This is because the class category is not selected in the pass settings under «Where can this pass be used?». Only passes and classes that are in the same category can be used together. Edit the pass, select the appropriate category and save the pass so that the class can be booked with this pass. See also: Category settings explained

If you offer livestream-only classes, i.e. the «Hybrid class» class setting is not enabled, make sure that the «Hybrid pass» pass setting is not enabled as well.