The debugging output indicates an issue with IS-IS level configuration on interfaces attempting to form an adjacency. For IS-IS adjacencies to form correctly, both interfaces must operate at the same level or be configured to support multi-level (Level-1-2). If one router's interface is set to Level 1 only while another's is set to Level 2 only, they will not form an adjacency because they are operating at different levels of the IS-IS hierarchy. References: Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) v1.1