Review cassette installation.
Based on your responses, your frame appears to be set up correctly on the CORE.
The next thing to confirm is that the cassette is installed squarely and without play.
It's best to remove your bike for this step.
With the bike removed, facing the cassette from the side of the trainer, place your hands on the larger cogs of the cassette and try to pull them toward you and push them away.
Is there any movement in the cassette in those directions?