Welcome to the forums,. newcomer!
The Dell OptiPlex 7010 was released about more than a decade ago, how old is the PSU in the prebuilt OptiPlex? Even if the PSU was brand new(out of the box) you don't have more than 240W of power at your entire system's disposal. You're advised to have a 350W or higher wattage, reliably built PSU to power the entire build. In it's current state I'd advise against dropping a discrete GPU in there. Instead save the funds and put that into a new build from the ground up.