I suppose it is because Friendica is trying to use some php functions that are disabled. I checked it and seems Friendica needs these functions:
pcntl_fork
pcntl_waitpid
pcntl_signal
exec
system
passthru
shell_exec
proc_open
Check if these functions are disabled in php 8.1:
grep disable_functions /etc/php/8.1/fpm/php.ini
If they are disabled, edit file /etc/php/8.1/fpm/php.ini
and remove them from disable_functions
directive and once done restart php8.1-fpm
:
Warning: disabling those functions could be a security risk.
systemctl restart php8.1-fpm
Now try to install Friendica again.