Did you fill in the coach skill fields ? There are five input fields that by default read 0 (out of 150). You need to input the skill values you want for the coach. If you are looking to hire a fitness coach, just type 150 in the fitness field and leave the rest to zeros. Once you've done this, the signing fee and weekly wage shown above the "hire" button should change and show you the cost of your coach.
Are you sure you want to start running a public gym ? I'm just asking, because it seems like you have hardly any in-game money, and gyms are very expensive to run and a challenge to just break even financially. Are you aware of the costs ?