Mors clanrats are only recruited as "limited time" offers, you have to activate their effect from mors forge panel and for next couple turns you can keep recruiting them

Took me a while to figure this out, too--the tooltips on the recruitment buildings for them have a small note in red. After you get the level 3 infantry building, Mors Clanrat recruiting has to be done through Queek's Awe system--on the Warcries tab, the top option Gnawdwell's Decree unlocks recruitment of Mors Clanrats for 5 turns at scaling Awe cost. The Red Guard Stormvermin recruitment are unlocked by a skill in Queek's skill tree.