error while loading shared libraries: libgfortran.so.3: cannot open shared object file: No such file or directory

Question

I obtain an error:

error while loading shared libraries: libgfortran.so.3: cannot open shared object file: No such file or directory

My OS:

$ lsb_release -a
No LSB modules are available.
Distributor ID: LinuxMint
Description:    Linux Mint 19.2 Tina
Release:    19.2
Codename:   tina

I tried:

1. Running:

   sudo apt-get install libgfortran3
   

   gives:

   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   libgfortran3 is already the newest version (6.5.0-2ubuntu1~18.04).
   0 upgraded, 0 newly installed, 0 to remove and 507 not upgraded.
   

2. Running:

   sudo apt-get install libgconf-2-4
   

   and obtained the same error.

3. Running:

   sudo apt install apt-file && sudo apt update
   

   gives:

   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   apt-file is already the newest version (3.1.5).
   0 upgraded, 0 newly installed, 0 to remove and 507 not upgraded.
   Reading package lists... Done
   E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
   E: Unable to lock directory /var/lib/apt/lists/
   

4. Running:

   sudo apt-get update
   

   gives:

   Reading package lists... Done
   E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
   E: Unable to lock directory /var/lib/apt/lists/
   

What should I do, please?

After advice:

sudo apt install apt-file && sudo apt update
Reading package lists... Done
Building dependency tree       
Reading state information... Done
apt-file is already the newest version (3.1.5).
0 upgraded, 0 newly installed, 0 to remove and 507 not upgraded.
Get:1 https://repo.skype.com/deb stable InRelease [4502 B]
Ign:2 http://packages.linuxmint.com tina InRelease                             
Hit:3 http://packages.microsoft.com/repos/vscode stable InRelease              
Err:1 https://repo.skype.com/deb stable InRelease                              
  The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
Hit:4 http://archive.canonical.com/ubuntu bionic InRelease                     
Hit:5 http://packages.linuxmint.com tina Release                               
Get:6 http://dl.google.com/linux/chrome/deb stable InRelease [1811 B]          
Hit:7 https://packages.microsoft.com/repos/ms-teams stable InRelease           
Err:6 http://dl.google.com/linux/chrome/deb stable InRelease                   
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4EB27DB2A3B88B8B
Hit:9 https://linux.teamviewer.com/deb stable InRelease                        
Hit:10 http://security.ubuntu.com/ubuntu bionic-security InRelease             
Ign:13 https://cloud.r-project.org/bin/linux/ubuntu tina-cran40/ InRelease     
Hit:14 https://cloud.r-project.org/bin/linux/ubuntu bionic-cran40/ InRelease   
Err:15 https://cloud.r-project.org/bin/linux/ubuntu tina-cran40/ Release       
  404  Not Found [IP: 65.9.95.64 443]
Hit:16 http://ppa.launchpad.net/ubuntu-desktop/ubuntu-make/ubuntu bionic InRelease
Get:12 http://ppa.launchpad.net/c2d4u.team/c2d4u4.0+/ubuntu bionic InRelease [15.9 kB]
Hit:17 http://archive.ubuntu.com/ubuntu bionic InRelease                       
Hit:18 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:19 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Err:12 http://ppa.launchpad.net/c2d4u.team/c2d4u4.0+/ubuntu bionic InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F142A4D99F16EB04
Hit:11 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
Reading package lists... Done 
W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
E: The repository 'https://repo.skype.com/deb stable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://dl.google.com/linux/chrome/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4EB27DB2A3B88B8B
E: The repository 'https://cloud.r-project.org/bin/linux/ubuntu tina-cran40/ Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ppa.launchpad.net/c2d4u.team/c2d4u4.0+/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F142A4D99F16EB04
E: The repository 'http://ppa.launchpad.net/c2d4u.team/c2d4u4.0+/ubuntu bionic InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.