You should never buy iPhones, iPads, or other Apple products without going through official channels; it's a guarantee you'll encounter problems and have no solutions to resolve them.
Hello, Did you buy it new? This kind of thing happens with iPads that are locked because they are reported stolen and sold second-hand on the street (scam), https://support.apple.com/fr-fr/HT201472. Otherwise, read and do what is indicated on this page and if it's not better, contact Apple or the carrier or the store where you bought this iPad. bazfile Moderator/Contributor security.