My usual advice would be to check that 0 is not being entered as O, but apart from that it's hard to say without knowing what the error message is.

Is this a boxed copy, or download? If it was download, you can log into Content Paradise or whichever reseller it was and retrieve the serial number from there. Copy and paste to make sure you've got the number correct.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire