I use php 7.4 Mariadb 10.5. without issues, I think there were some problems with 10.3 when I was doing migrations, but not sure about fresh install.
Softaculus makes installs very rarely right.
I need this to know in order to be able to help you:
Can you check if there are any errors?
(Right side gear icon / system info / log)
Is this an updated Mautic or fresh install?