Is it Ok to use GUID as the Foreign Key?
Using a UUID column as a foreign key is fine, and if that is the only candidate key on the parent record recommended/required. The FK must refer to a value that is unique in the referred table, this usually means the primary key though any column (or combination of columns) defined as unique by way of a constraint or index will do. Some suggest that it should be something that is always populated too (i.e. defined as not nullable), otherwise not all the rows in the referred table can be referred to by the FK, though there are many examples where this is not correct.

If the primary key on the referred table is a uniqueidentifier column then unless you have another unique value in that table you should use that as your foreign key target.

The only real reason that people might recommend against UUIDs as foreign keys is space: such values take 16 bytes to store instead of 4 (assuming the alternative is a 32-bit integer) or 8 (for a bigint).

There are arguments against using UUIDs as primary keys (though there are equally valid arguments for too!) especially if your PK is also your clustering key (i.e. the only member in or a significant member of the table's clustered index if it has one), but that is a different discussion.