The error dialog window usually indicates that you do not have permission to put certificates to the certificate store. That could be a security measure applied by your corporation administrators or by a 3rd party tool. Try to solve the issue using one of the workarounds provided here and here or consult your network administrators.
Every time I go to check Decrypt HTTPS traffic I get a pop up saying 'scary text ahead' then when I click yes it says it can't trust the fiddler root certificate, I've tried exporting it to desktop then manually trusting it but it's doing nothing.
Then I get this in the log.