Why whenever I try to sign a coach it says that I don't have enough cash flow? I took over a team and coaches really sucks so I'm trying to poach some coaches, I succesfully got my new head coach but I want to sign offensive and defensive coordinators but I always get this message: "you do not have enough cash flow to sign this coach".

I have 40 millions $ under the salary cap, ***?

Your cash flow for coaches is separate from your salary cap. You can see your cash flow situation when you make an offer by looking at the bar on the top of the form. You may need to first replace some overpaid coaches to free up space, especially if you have overpaid position coaches. But at the very least you should be able to pay an incumbent the same as the current coach is making (although the issue you might be running into is that this amount is not enough to poach the coach you are after)

I made an offer to a position coach, the old one was making 12 millions $ per year, I've offered 3 millions $ to the new coach but it still says I don't have enough cash flow. I also have a pending offer to an offensive coordinator.

If you have a pending offer that reduces your cash flow by that much then you should be ok. It may take 2 sims though, one to get your cash flow back down and one to make the new offers. But you can make the offer with that warning visible, whether it works just depends on the order the offers are processed (which is random).