HI @TRon, the attached screen shot has all the answers to your questions and also has the output after following your instructions and running make bigide.
As suspected Debian 11 which dpkg does not support that compression format. I don't believe it is back-ported either but not sure as I have moved on to debian 12 (bookworm) the moment bookworm became stable.
I have so many thing's on this system I am frankly surprsed it works at all conisdering the things I have done to it over time. Maybe it is time to get a new hard drive and start fresh?
In case it is not too much of a burden then yes, please start over with a fresh Linux install.
When time to add FPC and/or Lazarus, do not use the package manager or.deb files to install FPC/Lazarus but use FPCUPDeluxe instead. At least to install FPC into user-space (home directory comes to mind). FPCUpDeluxe's FPC installation makes sure that FPC is completely stand-alone by using a shell script and forced fpc.cfg configuration file. That way you could in the future install/use multiple versions of FPC. Only thing required is to have that FPCUpDeluxe fpc.sh script either in the path or create a shell alias or link in order to be able to start free-pascal from the commandline.
If that is done and works as expected then Lazarus can be build from source with "make FPC=/path/and/name/of/script/fpc.sh bigide".
If one sticks to that method of installation then any reference of default locations for fpc.cfg files and FPC searching units/sources in system directories indicate that something is amiss (instead of now having no clue what happened by whatever installation).
I have no clue as how to fix the current issues you are confronted with. I know how to work around it in case necessary but I would highly suggest to start anew with the things I wrote in mind.