gpg: signing failed: No secret key

[Robert J. Hansen]

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> gpg: key "6O0PDA84A36B6C98B261AC2020546703CDADFA53" not found

That's not a valid key ID.  Key IDs are strings of hexadecimal digits. 
Your second 'digit' there is the letter O, which is not a valid hexit.

> gpg --delete-secret-keys CDSXFA53

That's not a valid key ID.  Neither S nor X are valid hexits.
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