When you purchase a pre-owned phone from a source like Ebay it's always a matter of confirming the validity of the seller. This phone was obviously not checked out before they took your money.
No, it's not just that this won't be a 'simple' matter to clear up (if it was, there's no point in having any kind of parental controls to begin with). an important point is anytime you buy a pre-owned phone, you don't want to have anything tied the previous owner left on it. You want to have a phone that's clean the first time you start it up so you set it up the way you want, not how the previous owner had set it up. Also, you don't want anything like a spyware utility installed that the previous owner might have left behind.
Anyway, looking up the specs for this model, it's not very promising. Unless you have very minimal needs, it's usefulness as a daily usage phone will need be set by low expectations. It's a low-spec phone that will be running a dated, unsupported version of Android so as time goes on you will most likely have issues interacting with a lot of today's online services.
https://www.gsmarena.com/samsung_galaxy_j1_mini_prime-8397.php
With only 1GB of RAM you will experiencing a lot of buffering and pauses as you use it, and with only 8GB of storage you will be constantly and frequently having to deal with 'out of memory' pop up messages.
But if you want to spend the effort on resurrecting this phone, you can go here to find a stock ROM for this phone:
https://www.sammobile.com/firmwares/search/sm-j106/
Be sure to select the appropriate ROM that exactly matches your phone. Do not mix and match ROMs, they are not interchangeable. You need to determine which model of Galaxy Prime Mini you have (SM-J106B, F, H, or M) and download that ROM. When you select the ROM you need, the download page has instructions you need to follow to do the actual flashing process in the right column.
I'd still recommend you just return this phone and get one where you don't have to go through all this hassle to just get it workable again.