Hi there, you can back up your computer and wipe your Macintosh HD in macOS Recovery Mode, what followed is to reinstall your Mac from the backup that you've made. Then, you can upgrade macOS Mojave to macOS Catalina without any barrier.
After the formatting, you can restore your Mac via Time Machine. All the applications that you download and the files that you save will be on the original location.
Till now, you can go to Software Update to update your Mac from macOS Mojave to macOS Catalina.