I am in the process of changing sprockets and chain.

When I got to taking the rear wheel off, trying to loosen the nut did not loosen the nut. Checking the other side of the axle confirmed that it's just spinning the axle as a whole. It even started rotating the metal plate that's for the chain tension bending it and scratching my swing arm in the process. If anyone has faced a similar problem or has any insights, I'd highly appreciate your help. Thank you!