If you mean it says serial key not valid,then only the usual advice.Or do you get some other error?
Zero's and noughts,ones and i's and sometimes v's and u's can be misread .Sometimes even z's and 2's and L's and 1's depending on the type and print.All these need double checking.
Also expansion pack keys aren't valid.(i assume it is the full game)
Sorry if this is stating the obvious.
Hopefully,a useful link.
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