Here is theory of One For All which shows why Monoma couldn't use its Power after copying it

One For All Theory

While OFA is often regarded as the amalgamation of all the collected abilities under its umbrella, the stockpiled power is not the actual OFA ability. The actual OFA is just the ability to transfer your quirk. The reason Monoma couldn't do anything with it is because of the stockpiled power aspect takes time. 

The longer a person has the quirk, the more power they build up for future use. So, of course, he couldn't do anything in 5 or 10 minutes unless the only thing he wanted to do was transfer his copying quirk to someone else (but he'd have to know about that and actually want to do it). The difference between inheritance and copying of the quirk is that inheritance gives everything, including the stored power. This is why Izuku was able to summon All Might-level strength in a short time after receiving the quirk, but Monoma couldn't do anything.