Well, for starters, you could provide enough information for us to do an initial assessment.
For each of these questions, please respond with only full and proper part numbers. No descriptions!
What NUC model?
What memory (SODIMM(s)) have you added?
What storage (HDD(s), SSHD(s), SSD(s)) have you added?
What keyboard and mouse are you attempting to use?
What operating system are you attempting to install?