Hi there, provided that you got an external boot hard drive with macOS Sierra, you can repair the internal hard drive with it for sure. As long as you boot your device from the external hard drive, go to Utilities > select Disk Utility > run First Aid to repair the internal hard drive.
When the repairing process ends, you can upgrade your Mac from macOS Mojave to macOS Catalina without a hitch. This article How to update your Mac coming from iBoysoft tells you how can you upgrade your macOS in detail.
