Helo @Fotofobia and welcome to the community.
Starting with MySQL 8 you no longer can (implicitly) create a user using the
@Fotofobia, you have chosen Ubuntu 20.04 LTS upon which to install Virtualmin and you have MySQL 8. It also includes PHP 7.4, and I guess these are newer versions compared to what you had running on cPanel, hence the errors related to compatibility. Had you chosen an OS and version which matched with the OS and version of MySQL and PHP which ran on your cPanel system then you might not have had to deal with errors such as these.
The easiest migration path for you would be to install Virtualmin on a system which is identical to the one you are migrating cPanel from, else you will have to understand and solve compatibility issues which may crop up at the time of migration or further down the line.
I am not saying it can’t be done or that a future version of Virtualmin might not handle this gracefully but at present you had better use an identical system for migration.