I have like 5 more coaches to hire. There is an error message when I try today. Will I be able to hire coaches before the training camp?

Not sure what the reason for the error message now is, but you ought to be able to hire them during any of the late season free agency periods. I also think that any offers submitted before the end of the draft will process either when it concludes or during LSFA-1 (it's been a while since I've gone that far without refilling the staff, so I can't remember when they pop in...)