Can't install new software in the server

Question

I'm trying to install vsftpd on Ubuntu 18.04 LTS, but when I run apt-get install vsftpd I get this error:

You might want to run 'apt --fix-broken install' to correct these.

The following packages have unmet dependencies:

linux-image-generic : Depends: linux-image-4.15.0-42-generic but it is not going to be installed

Recommends: thermald but it is not going to be installed
linux-modules-extra-4.15.0-39-generic : Depends: linux-image-4.15.0-39-generic but it is not going to be installed or

linux-image-unsigned-4.15.0-39-generic but it is not going to be installed

linux-modules-extra-4.15.0-42-generic : Depends: linux-image-4.15.0-42-generic but it is not going to be installed or

linux-image-unsigned-4.15.0-42-generic but it is not going to be installed

E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

I've tried run apt --fix-broken install and apt-get install -f vsftpd but with no sucess.

I looked these answers but the solution didn't worked for me: Can't install new software/packages New software can't be installed: Dependency Problems

Update:

df -h output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        39G  7.3G   30G  21% /
devtmpfs        989M     0  989M   0% /dev
tmpfs           992M     0  992M   0% /dev/shm
tmpfs           992M  680K  992M   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           992M     0  992M   0% /sys/fs/cgroup
/dev/xvda1      232M  224M     0 100% /boot
tmpfs           199M     0  199M   0% /run/user/0

df -i output:

Filesystem      Inodes  IUsed   IFree IUse% Mounted on
/dev/root      2537760 337826 2199934   14% /
devtmpfs        253100    434  252666    1% /dev
tmpfs           253889      1  253888    1% /dev/shm
tmpfs           253889    606  253283    1% /run
tmpfs           253889      4  253885    1% /run/lock
tmpfs           253889     18  253871    1% /sys/fs/cgroup
/dev/xvda1       62248    326   61922    1% /boot
tmpfs           253889     10  253879    1% /run/user/0

apt-cache policy linux-image-generic output:

linux-image-generic:
  Installed: 4.15.0.42.44
  Candidate: 4.15.0.42.44
  Version table:
 *** 4.15.0.42.44 500
        500 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     4.15.0.20.23 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
     4.4.0.139.145 500
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages

uname -r output:

4.15.0-36-generic

Update 2: I try this command:

dpkg -l linux-image-\* | grep ^ii

it shows only one kernel:

ii  linux-image-4.15.0-36-generic          4.15.0-36.39  amd64        Signed kernel image generic

When I try to remove:

apt-get remove linux-image-4.15.0-36-generic

The return is:

ou might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 linux-image-generic : Depends: linux-image-4.15.0-42-generic but it is not going to be installed
                       Recommends: thermald but it is not going to be installed
 linux-modules-extra-4.15.0-36-generic : Depends: linux-image-4.15.0-36-generic but it is not going to be installed or
                                                  linux-image-unsigned-4.15.0-36-generic but it is not going to be installed
 linux-modules-extra-4.15.0-39-generic : Depends: linux-image-4.15.0-39-generic but it is not going to be installed or
                                                  linux-image-unsigned-4.15.0-39-generic but it is not going to be installed
 linux-modules-extra-4.15.0-42-generic : Depends: linux-image-4.15.0-42-generic but it is not going to be installed or
                                                  linux-image-unsigned-4.15.0-42-generic but it is not going to be installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).