The restriction is based on salaries on your team. You can, for example, have:
HC : 100,000
Coords: 99,999
Position: 99,998
There is a bug with the AI that doesn't follow that if it signs a coach for you, in those cases you can rebalance your coaching salaries, if you get that message and perform the rebalance it will adjust your salaries so that the HC makes twice the coordinators who make twice the position coaches. This bug should be fixed in the next general release.