Yellow Triangle on C Drive after Disabling Secure Boot

Run this command from elevated command prompt:

manage-bde -off C:

This will disable Bitlocker protection for your system drive. You can check your status with this command:

manage-bde -status C:

What does this mean?

It means Device Encryption is suspended.

Will this affect anything?

It only means while your files are encrypted the encryption key is accessible. However, Device Encryption should be disabled, since Ubuntu does not play well with it.