I'd also be very surprised if the 3d printed part wasn't fitting properly. If it feels like it's more than just machining burrs that are keeping it from fitting properly, then it's likely not assembled correctly. This stuff is all CNC milled, 3d printed, etc. Don't anticipate much room for major error. Are you certain that you have all three of those pieces (the ones that go inside of the upper) inserted correctly?
Here's a BASIC outline of assembly. There's a lot of fit and finish before reaching this stage, as I mentioned (deburring, removing CNC machine lines, blending certain transitions that the CNC couldn't handle, adding slight chamfer to sharp edges to mimic hand-finished look, etc. Also, there's a fair amount of nuance to bluing/painting/staining/oiling/weathering these parts to achieve the desired finish. Not gonna go into that here.
First, FIRE BLUE the following parts: 13, 15, 20, 25, 27, 28, 29, 31
Attach bolt-stop to upper with 2 pins
Assemble/attach sight assembly to upper
Attach grips
First, hold the upper upside down and drop the bolt into place inside of it, then drop in the bolt spring. Then, slide piece #32 into place. Make sure you have it oriented the correct way, and it should slide right under the "rails" and all the way back to the rear of the upper. It should slide back and fit very well and snug against the rear of the bolt. Then, slide in the aluminum piece (#33) making sure it's oriented the right way. The 3d printed piece should then just drop right in. It has to be oriented correctly top to bottom, but I don't think the back-to-front orientation matters with this piece. Keep this whole assembly upside down on your bench so that the pieces don't fall out.
Next, assemble the mag plate with button, spring, set screw, etc.
Next, drop the trigger/pin into the lower so that it sits PROPERLY. You have to rock the trigger back and forth a bit in order to get the pin to sit fully in the milled recesses. I drop it in with some thin needle-nose pliers, then hold the trigger with my hand from the outside and guide it into place. Holding the lower upright so that the trigger doesn't fall out of place, slide the assembled upper onto the lower (also making sure that the upper's inner pieces don't fall out.) CAREFULLy rest the whole thing on your bench, on its side, so that nothing falls out.
Next, assemble the internal mechanism. I apply grease to all pins, springs, anywhere that has metal moving against metal. I also mocked up a super thin plastic washer (like the thinnest plastic you can find >1mm thick from a blister pack, etc.) to fit under the safety to tighten that up. Once that mechanism is assembled you'll have to hold it together tightly with your hands so that the springs don't pop out, shoot across your room and get lost forever. Cock the hammer back, then (at a slightly pitched-forward angle) insert the whole mechanism into the back of the lower (holding the mechanism together tightly with your hands for as long as you can, so that it doesn't pop apart inside of the gun). Once it's sunk mostly in, level it out and push HARD until you hear a subtle click/see the release lever snap downward into place.
A few pulls of the trigger/cocks of the hammer and the grease should disperse, leaving you with smooth snappy action.
A few things to note:
Attach the scope mount AFTER the Mauser is assembled. The rear bit of all-thread, when threaded all the way in, will put pressure against the firing mechanism and won't allow it to enter or exit the lower smoothly.
The bull barrel should be attached AFTER the Mauser is assembled / removed BEFORE it is disassembled. You won't be able to slide the upper on/off with the bull barrel attached.