The CLA Assistant appends each signature as a direct commit to the branch named in 'branch:'. That was main, which the 'Require CI on main' ruleset rejects ('Repository rule violations found'), so signatures were never recorded and the check stayed red however often a contributor signed — a deadlock for every outside contribution.
Point the store at the cla-signatures branch. The ruleset targets the default branch only, so the bot can append there without granting any actor a bypass on main. That branch is seeded with the existing signature and carries its own ruleset blocking deletion and non-fast-forward pushes.
TheZwiss