I didn't open your model, but it sounds like you need to fix your constraints. Soft springs can sometimes help when it is difficult to properly constrain a model, but they should be a sort of last resort. Check your constraints to make sure that you have stopped all six degrees of freedom. If you haven't, then come up with the missing constraints. If you are having trouble coming up with a realistic constraint, you could at least try adding a less realistic constraint to make sure that you've found the problem.
I downloaded your model and I see what the problem is. This is a big simulation because there are so many bodies in it. Chances are, there are some bodies (for whatever reason) which are not being restrained properly. Going back to Newton's Laws, an object in rest will stay at rest unless acted on by an outside force. In this case, if a panel isn't being constrained properly and there is a force acting on it, that panel will fly off the screen and you'll get a message that says you don't have adequate fixtures.
In order to get around this, you'll have to change your workflow. Start off with a few bodies (4 at max) then create your connections. Apply loads and fixtures and make sure all the bodies stick together. The loads and the fixtures aren't important at this stage, just make sure that the simulation runs and the bodies stick together. If it runs, add a few more bodies, repeat. Do this until all the bodies are added into the simulation. Then, focus on accurate loads and fixtures.
I know this sounds like it will take a long time, but it's worth it and this is the only way to do a simulation like this.
All the best,
Good advice by Nicholas Luyster. In addition (and in conjunction with his advice) I would reduce your element size significantly so that your analysis runs faster. I like to run with a draft mesh when starting out, if possible. It will be inaccurate but you'll quickly be able to see that it runs correctly as you add more bodies and more restraints. Once you successful prove out your constraints and loads then you can refine the mesh for higher accuracy. It will run longer but you could set it to run overnight.
make sure you're also using the right idealization
thick and blocky = shells
thin flat = shells
long and constant cross section = beams