Mails do not get signed with DKIM in aapanel

I have successfully added the domain for emails in aapanel and I can send the emails. But when I test the mail with it shows the email is not signed with DKIM.

How do I solve the problem?

I had the same problem. The issue was the server cannot load dkim key file.
There was a permission issue:
dkim_module_load_key_format: cannot load dkim key /www/server/dkim/ cannot map key file: '/www/server/dkim/' Permission denied

All I had to do was set permission from 640 to 644, then restart rspamd
You can try this command below.

chmod 644 /www/server/dkim/
/etc/init.d/rspamd restart

Or you can just browse the file from aapanel file explorer and change the file permission.