That means /usr/lib/exim4/exim4 doesn’t exist or is not executable.
ls -l /usr/lib/exim4/exim4
ls -l /usr/sbin/exim4
Wait a moment, you removed exim, I didn’t see it before:
root@na7:/var/log/exim4# dpkg -l | grep -i exim
rc exim4 4.96-15+deb12u2 all metapackage to ease Exim MTA (v4) installation
rc exim4-base 4.96-15+deb12u2 amd64 support files for all Exim MTA (v4) packages
rc exim4-config 4.96-15+deb12u2 all configuration for the Exim MTA (v4)
rc exim4-daemon-heavy 4.96-15+deb12u2 amd64 Exim MTA (v4) daemon with extended features, including exiscan-acl
root@na7:/var/log/exim4#
rc means: r removed and c config files still there.
ii exim4 4.96-15+deb12u3 all metapackage to ease Exim MTA (v4) installation
ii exim4-base 4.96-15+deb12u3 amd64 support files for all Exim MTA (v4) packages
ii exim4-config 4.96-15+deb12u3 all configuration for the Exim MTA (v4)
rc exim4-daemon-heavy 4.96-15+deb12u2 amd64 Exim MTA (v4) daemon with extended features, including exiscan-acl
ii exim4-daemon-light 4.96-15+deb12u3 amd64 lightweight Exim MTA (v4) daemon