Lightroom 6

"Serial number is invalid".

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If you're seeing that message then you've downloaded and started installing.

Where did you purchase Lightroom 6?

Without posting the serial number here, are you entering a 24 digit number?

Check these tips from Adobe

https://helpx.adobe.com/download-install/kb/find-serial-number.html

Are you downloading https://techterms.com/definition/download
or Installing https://techterms.com/definition/install ???

I don't understand your question.

• Do you have the original disk or download installer you received at purchase?
• Do you have an old operating system that can support legacy LR6?  
• Did you purchase directly from Adobe?
• Can you access your serial number from your online account below?
  https://account.adobe.com/products