There are several factors that will come into play here.
First off, check your Finances>Financial Fair Play tab. Is your Projected Yearly Player Wage Expenditure close to or above your Allowed Yearly Player Wage Expenditure? If so, that could be why.
Also, any incoming transfers and players on loan will be calculated into future salary expenditure. So if you have players on loan that, for example, have combined wages of £100k p/w, that will be calculated into what you can afford to pay a new player on his new contract.
I'm sure you have players above £100k p/w if you're trying to get Pogba. Check your Finances>Salary tab, and see what you're allowed to pay players in the different squad status categories. If the Backup, Rotation and First Team categories have very low Allowed wages - which I'm sure they do - it could indicate your finances aren't as robust as you may think.
The FFP regulations state that wages cannot be increased by more than £4m per year. Only if your total wage expenditure is less than £60m are you exempted from that. Any team with a total yearly wage expenditure of more than £60m, have to abide by those rules.