Charles Weyland,

вам нужна multi-statement функция:

https://docs.microsoft.com/ru-ru/sql/t-sql/statements/create-function-transact-sql?view=sql-server-ver15 -- Transact-SQL Multi-Statement Table-Valued Function Syntax
CREATE [ OR ALTER ] FUNCTION [ schema_name. ] function_name
( [ { @parameter_name [ AS ] [ type_schema_name. ] parameter_data_type
[ = default ] [READONLY] }
[ ,...n ]
]
)
RETURNS @return_variable TABLE <table_type_definition>
[ WITH <function_option> [ ,...n ] ]
[ AS ]
BEGIN
function_body
RETURN
END
[ ; ]

А вы пытаетесь сделать табличную in-lene