It can be a major problem and can ruin the canner so my first option would be returning it for a refund.

If that isn't possible a couple of things you can try but with no guarantee they will work. If the gasket, the rubber seal is damaged/destroyed you can try inject some WD-40 up into the seal area and between the rim where the metal slides against metal. Inject it in several places with the canner inverted so it will drain into the seal area. Then try carefully tapping on the handle to get it to move toward the release point. You may have to inject more silicone spray as you work on it.

The other option is to place the canner in the freezer or submerge the canner is ice water for several hours (but keep the gauge above the water level). Let the canner get really cold, cold enough that the metal begins to contract. Then see if the lid can be removed.

Lastly, find a local machine shop. They usually have clamp wrenches that are large enough to encircle the entire rim and can torque it off, hopefully without any permanent damage.

Either way if you get it removed examine the entire area for damage, replace the gasket, and give it a trial run with 3 inches of water in the bottom. As it begins to vent steam check the entire rim area for leaks.

Good luck

Dave