If mono --version shows less than 5, try update from https://www.mono-project.com/download/stable/#download-lin-raspbian and if that doesn’t help see FAQ: Security to make sure certificates are correct.
If mono --version shows less than 5, try update from https://www.mono-project.com/download/stable/#download-lin-raspbian and if that doesn’t help see FAQ: Security to make sure certificates are correct.