This statement made the Developers role the owner of the Test schema. In being the owner, any grants you attempted to do that overlapped the permissions afforded by schema ownership would fail:

ALTER AUTHORIZATION ON SCHEMA::[Test] TO [Developers]

As a side note, CREATE TABLE and CREATE PROC are database-level permissions and therefore cannot be granted at the SCHEMA level. The CRAETE TABLE plus ALTER SCHEMA privileges are required to create tables in a particular schema, although schema-ownership supercedes the need for the explicit ALTER SCHEMA permission.