hi there got a new donation put in my mac address an i keep getting error can sum1 help me wit this please

hi there i tried to asign my mac addy an its sayin [the following MAC Address(es) already exist in our system or are invalid.] can sum1 please help me wit this matter.Thanks for your help in advance

Heres the deal my next door neighbor bought me a android box to put iptv on for them because they didnt know how. i didnt do it rite away and forgot which box they wanted it to be installed on so i end up setting a mag 254 and it worked fine except one thing its wasnt the box i was suppose to...