Help with 75243 Slave 1. Parts not fitting into eachother Help!

I do not know if this is the right sub, however I will try my luck.

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I recently bought the 75243 Slave 1 for my fiance and me to build together. We just made it to step 191 in the manual where we are supposed to fit to pieces into each other (4211888 and 395726 for reference). There is no way this is possible and we are more likely to break one of the parts if we keep on trying.

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What do we do?