Hi, Christine-

Thanks for posting the system check output.

One thing to note-- unrelated to the R stuff-- is this pair of error messages:

3dSkullStrip : FAILURE
3dSkullStrip: error while loading shared libraries: libgsl.so.19: cannot open shared object file: No such file or directory
3dRSFC : FAILURE
3dRSFC: error while loading shared libraries: libgsl.so.19: cannot open shared object file: No such file or directory

I think that reflects that one small step in installation was missed. Could you please copy+paste this into a terminal:

sudo ln -s /usr/lib/x86_64-linux-gnu/libgsl.so.23 /usr/lib/x86_64-linux-gnu/libgsl.so.19

... which will ask you for your admin/root password. Could you then open a new terminal, run the system check again, and verify that the above errors are gone?

Re. the R package installation errors: sigh, R has been a bit of a pain... I notice that you have R versions 4.0. At the moment, AFNI requires 3.6.*, and I though that the latter was the version of R installed by Ubuntu 20.04's package manager (Aptitude). Is the version of R that you have just from the initial big command here:
[afni.nimh.nih.gov]
... or through some other installation means?

--pt