Okay let me get this understood...
You have installed retropie on a Ubuntu 20.04 based 64 bit PC not a raspberry pi correct?
1. How did you install the emulator?
2. What is your default emulator?
(I'm assuming VICE as it is the default.)
RetroPie install on Ubuntu 20.04 x64 ( Vic-20 runs fine from RetroPie)
So it loads just fine...
try this instead:
# Generated by Attract-Mode v2.6.1
#
executable /opt/retropie/supplementary/runcommand/runcommand.sh 0 _SYS_ vic20
args "[romfilename]"
If i recall correctly that is what I had to do...do you have any other systems running on your setup through AM now?
also check /dev/shm/runcoomand.log and see if there are any errors reported.