See the AFNI project page. To use AFNI, you’ll use the module tool.

You can see what versions are available by using:

[me@login01 ~]$ module avail afni

------------------------------ /opt/modulefiles -------------------------------
   afni/20.3.03

Use "module spider" to find all possible modules and extensions.
Use "module keyword key1 key2 ..." to search for all possible modules matching
any of the "keys".

To load a specific version, you would use:

[me@login01 ~]$ module load afni/20.3.03

while the "afni" wildcard will load the default version, afni-20.3.03 in this case.

You should now be able to run AFNI commands:

[me@login01 ~]$ afni --version

Precompiled binary linux_openmp_64: Dec  7 2020 (Version AFNI_20.3.03 'Vespasian')"