You can’t use a function to insert data into a base table. Functions return data. This is listed as the very first limitation in the documentation:
User-defined functions cannot be used to perform actions that modify the database state.
“Modify the database state” includes changing any data in the database (though a table variable is an obvious exception the OP wouldn’t have cared about 3 years ago – this table variable only lives for the duration of the function call and does not affect the underlying tables in any way).
You should be using a stored procedure, not a function.