LibreOffice can't see my PGP keys or find them wrong

I am using LibreOffice (french version on Ubuntu). I don’t see how to digitally sign a file. When I select digital signature, I don’t see any signature. When I try to save a file with the GPG button selected, the public keys can be seen, but when I chose one, I have a error message, the key is wrong, damaged or encrypting can’t be done.