Instalation error: Public key not signed on elementaryOS

[ChaboCode]

How to reproduce

• Using elementaryOS 5.1.6
• Using installation instructions for Ubuntu 18.04
• Got GPG error: repository not signed
• Delete /etc/apt/sources.list.d/mono-*
• Using installation instructions for Ubuntu 16.04
• Got the same error

[ChaboCode]

if someone have the same error, it can be because of bad keys. I removed that keys and it worked.