I got the key from Fanatical's Mystery Star Bundle. The site has a help section for when Steam says the key is invalid. Can you please try this?: (relevant section copy-pasted from https://support.fanatical.com/hc/en-us/articles/201741151-Steam-reports-key-is-invalid)

Steam sometimes reports a key is invalid which it then later accepts.
If your key is reported to be invalid, please:

-Logout from the Steam client.
-Log back into the Steam client, take care to use the correct account and preferably wait at least a few minutes before logging back in.
-Try activating the key again.

Remember to include the "-" characters and enter letters in UPPER case. Ideally, use copy and paste through the clipboard, to avoid the possibility of typing mistakes and confusing "I" with "1" and "O" with "0".

If it still doesn't work, please tell me so I can open a ticket.