Thanks a lot, both of you!
I installed php 7.3 and managed to install both dovecot and Roundcube. I set the mail server and a user. Now the problem is when I try to login. It says

I googled the issue, and apparently it has to do with the dovecot installation.
When I run "systemctl status dovecot.service" I get the following:
dovecot.service - Dovecot IMAP/POP3 email server
Loaded: loaded (/lib/systemd/system/dovecot.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Tue 2021-07-27 06:18:03 UTC; 35min ago
Docs: man:dovecot(1)
http://wiki2.dovecot.org/
Main PID: 42407 (code=exited, status=89)
Jul 27 06:18:02 pkad systemd[1]: Started Dovecot IMAP/POP3 email server.
Jul 27 06:18:03 pkad dovecot[42407]: doveconf: Warning: /etc/dovecot/conf.d/10-ssl.conf line 20: Global setting ssl_cert won't change the setting inside an earlier filter at /etc/dovecot/conf.d/10-ssl.conf lin>
Jul 27 06:18:03 pkad dovecot[42407]: doveconf: Warning: /etc/dovecot/conf.d/10-ssl.conf line 21: Global setting ssl_key won't change the setting inside an earlier filter at /etc/dovecot/conf.d/10-ssl.conf line>
Jul 27 06:18:03 pkad dovecot[42407]: Fatal: service(lmtp) access(/usr/lib/dovecot/lmtp) failed: No such file or directory
Jul 27 06:18:03 pkad dovecot[42407]: master: Fatal: service(lmtp) access(/usr/lib/dovecot/lmtp) failed: No such file or directory
Jul 27 06:18:03 pkad systemd[1]: dovecot.service: Main process exited, code=exited, status=89/n/a
Jul 27 06:18:03 pkad systemd[1]: dovecot.service: Failed with result 'exit-code'.
Jul 27 06:28:57 pkad systemd[1]: /lib/systemd/system/dovecot.service:19: PIDFile= references a path below legacy directory /var/run/, updating /var/run/dovecot/master.pid → /run/dovecot/master.pid; please update the unit file accordingly
Any clue what the problem might be?
Thanks again!