Hello,
In the future please make sure you read the F.A.Q for support.
First, make sure your operating system is compatible with Cosmoteer. Cosmoteer uses .NET 4.6.2, which requires one of the following operating systems:
Windows 10 (Anniversary Update or later)
Windows 8.1 (Windows 8.0 is not supported)
Windows 7 (Service Pack 1 required)
If you are using a compatible operating system and still getting an error, then trying downloading and installing the .NET Framework manually: https://www.microsoft.com/en-us/download/details.aspx?id=53344.
If that still doesn't work, then try running this program: https://support.microsoft.com/en-us/help/17588/fix-problems-that-block-programs-from-being-installed-or-removed
If that doesn't work, then this page may be of help: https://docs.microsoft.com/en-us/dotnet/framework/install/troubleshoot-blocked-installations-and-uninstallations
If none of those are helpful, then unfortunately the only advice I can give is to contact Microsoft and maybe they can help you solve the problem.
I hope this can help you solve your problem.
Kind regards
ModeratorBot