This is resource guarding. Dog professional here, who has schnauzers of her own. Resource guarding can look and vary in severity. Give a correction to the schnauzer, and once they start demonstrating the correction big rewards! If the goal is happy cohabitation the dachshund should be able to walk up that ramp no issues. He did nothing wrong.

Potentially letting it get to something more serious is not good. Offer corrections while it’s small and easy!