Hi Pin! Can you make sure you're not copying any space at the end or the beginning of the key? Keep also in mind that you only need to copy the key up to the "==" part at the end.
Also, have you followed the recommendations in the article Invalid SSH Key? If the previous doesn't doesn't work, you can also try Invalid SSH key SSH keygen.
If that still doesn't solve it, you can run this in your terminal to see if this key is already in use in another account:
ssh -Tv git@bitbucket.org
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