I dont play CaC and just recently learned that it is a shortname for Create a Club.
I am guessing from what i have seen playing as a manager for a Danish midlevel team [1], i dont think there is a description in the game and i dont remember it mentioned in the manual (and i couldnt find it by browsing through the manual guickly).
It seems to me that the staff must have roughly the same level and potential as the player to make the warning disappear, so if the team on average have 3 gold stars and level 65 your staff should be of roughly the same quality.
It seems logical that the board want staff and players of roughly the same quality, it wouldnt make sense for a superstar player being trained by a second class coach or playing with players of much lower level (unless its a young up-and-coming player).
[1] The board isnt the only ones making demands, in the second season with the midlevel team i play, i am able to buy Beckham but he wont negotiate a new contract unless i get a striker at roughly his level (its the same if i buy Akinfeev as goalkeeper).
In contract-negotiations i have seen players complain about a couple of things, for example if the stadium are smaller than they are used to play at or the supporting infrastructure is not of a high enough standard (normally they forget about it if they get a big enough transfer fee or bonus, but i think it is a good idea to remember what the players complained about and fix it before you extend the contract next time).