I could install PHP 5.6 on Almalinux 8.10 without any issues:
# pwd
/opt/plesk/php/5.6
# ll
total 24
drwxr-xr-x. 2 root root 4096 Sep 11 22:02 bin
drwxr-xr-x. 3 root root 4096 May 29 09:18 etc
drwxr-xr-x. 3 root root 4096 May 29 09:18 lib64
drwxr-xr-x. 2 root root 4096 Sep 11 22:02 sbin
drwxr-xr-x. 7 root root 4096 Sep 11 22:02 share
drwxr-xr-x. 4 root root 4096 Sep 11 22:02 var
So, what's going on that you can't do this? Have you already contacted Plesk Support so they can investigate the issue on your server? It'll cost you nothing and it's the best service I've ever seen in my career.