As said in https://github.com/matomo-org/matomo/issues/16743, this has little to do with Matomo, but your MySQL setup seems to be broken in some kind if MySQL doesn’t allow you to create a MySQL user.
I’d recommend to check the documentation of your MySQL distribution or Linux distribution on how to set up MySQL with InnoDB.
Yes, I understand. But you suggested me to use this forum
If I check my MySQL server (“show engines”), I can see that InnoDB is listed and set to default.
I understand that this is a MySQL problem, but maybe someone knows what to do.
According to this answer, I think the following happened (keep in mind that I am not a database expert)
MySQL stores its data (like user accounts, etc.) in an own database.
Someone had a backup of an old version of MySQL where this database still used MyISAM. Then they restored that backup in a modern MySQL version which uses InnoDB by default and doesn’t support MyISAM anymore for this system table.