INSERT AFTER trigger failing with multiple rows
Try taking out the VALUES clause in your final insert (See the commented line below which give your error) and replace it with a simple insert (see the final insert below).  

Declare @Tb_Permissions table (LnkUserID int)
Declare @Inserted table (ID int)
insert into @Inserted(id) values(1),(2)
--INSERT INTO @Tb_Permissions ([LnkUserID]) VALUES ((SELECT [ID] FROM @Inserted));
INSERT INTO @Tb_Permissions ([LnkUserID]) SELECT [ID] FROM @Inserted;