Can't Select All Files on Ubuntu 20.04 Desktop and other anomalies, need your help ASAP

Question

I just installed Ubuntu 20.04 and a lot of weird things start to happen including: 1. Files from my previous OS (xubuntu) were not deleted, although i've deleted and created new partitions while installing Ubuntu 20.04 and 2. I can't select all files or all folders on Desktop! I can do it within folders but I can't use for example Ctrl + A on desktop, I can't even select all the files with a cursor, it only selects one! Please help me.