Keep in mind that it’s really not a slide assembly but an upper with a slide within. So once the upper fits like a normally seated slide the actual slide (bolt) needs to be pulled back to the little witness mark in order to seat the stop pin fully.
The instructions will show this more clearly, as well as the possible 3 places where a little bit of filing may be required. The one below the rear of the barrel isn’t quite as easily noted but there is a little ridge built up on one side where it’s intended to be adjusted, if necessary.