well, all feel to be at the center of europe : - )
except those who feel forgotten at its borders.
first, 1.4.8 is not compatible with 1.5.x (this means that if you had 1.5.x in the past, you won't be able to open your files with 1.4; of course 1.5.x can open files from 1.4.

if you use an appimage, you should use the latest one. 1.5.5. not 1.5.3 nor 1.5.1. if you have a bug that affects you, you should go for the latest 1.5.6svn and update it regularly.
also: you don't seem to be specially comfortable with the command line: you should probably install your software with a GUI tool...
this link
https://community.linuxmint.com/tutorial/view/1525 suggests that linux mint has a "software manager". you might want to use it.
apt is very powerful but it also gives you more chances to screw up everything.
further one: why aren't you installing mint 20?
ok, it's not a good number, but trying to be behind it's mostly the source of hassle.
ok, the fact that gimp might not be available is a bit scary...
https://en.wikipedia.org/wiki/Linux_Mint_version_historyfinally, in order to start scribus from a terminal (and getting a clue of why it does not start) you simply type scribus and then return in a terminal.
there is one last information that could help sort out the issue: if you type "which scribus" in a terminal, what does it spit out?