I swapped my engine and replaced bank 2, 3 codes popped up and the mechanic was able clear 2.. I still have the P0021 code popping up and my car is shaking and has misfire . is anyone familiar with the code or know what the problem is?

 

Hard to determine the issue with so little details, year, engine, who did the swap, verify all harnesses are clean and nothing damaged. Did you have the error with the PREVIOUS engine, what was the reason for the swap, etc.