Wordpress on ubuntu server

I have a physical server at my house. I have downloaded and installed ubuntu 20.04 and have configured a static ip. I have downloaded and installed apache2 & mariadb & php. I have downloaded & installed wordpress.

The funtionality of wordpress does not work correctly. I can not add themes or plug ins or back up site. The site does not work correctly. I can not figure out if it's a server error or WordPress error.