Table Function: Select statement with tablename in variable - Error: scalar type is not allowed

I have two SQL statements which are almost identical. The only difference is the table to be queried.

So I thought about using a Table Function (Table User Defined Function), (Table UDF) and passing the table name as a paramter:

FUNCTION "SCHEMA"."package::Function" ( in tbl varchar(200) )
...
Select col1, length(col1), left(col1, 6) ...
from :tbl

Unfortunately I get an error during activation:

Could not create catalog object: scalar type is not allowed; line 18 col 7

The same activation error (scalar type not allowed) occurs in a stored procedure.

So what exactly is the problem - and how to fix it??