ERROR 1064 (42000): You have an error in your SQL syntax - Cannot understand what the error means
I'm not an expert in Mariadb but I think you cannot do it. From the documentation on generated columns

Generated columns can be referenced in the INSERT, UPDATE, and DELETE statements.
However, VIRTUAL or PERSISTENT generated columns cannot be explicitly set to any other values than NULL or DEFAULT. If a generated column is explicitly set to any other value, then the outcome depends on whether strict mode is enabled in sql_mode. If it is not enabled, then a warning will be raised and the default generated value will be used instead. If it is enabled, then an error will be raised instead.