The filename is case sensitive. When you tried to --clearsign it you used the filename with an "Of" capitalized "UbuntuCodeOfConduct", while the filename stored in the disk has it in lower case "UbuntuCodeofConduct"
The filename is case sensitive. When you tried to --clearsign it you used the filename with an "Of" capitalized "UbuntuCodeOfCo nduct", while the filename stored in the disk has it in lower case "UbuntuCodeofCo nduct"