Hey there! I’d try the previous suggestions. If you’re still having troubles with that error message:
Delete the current “Install Mac OS Mojave” from your Applications folder, then download and run it again:
https://support.apple.com/en-us/HT210190
(The second link under section 4)
If none of those steps fixed it, I have seen the same installer work fine in safe boot on some Macs, so I’d try booting to same mode and opening the “Install Mac OS Mojave” again from Applications:
https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac
Ive seen that work particularly with Mojave for one reason or another.
If that were to also fail, (Hopefully not):
Id create a new administrator account in System Preferences > Users and Groups > (Unlock) “+”, restart, log into the new user then install it from there.
Also since you do already have High Sierra, you could boot to internet recovery, as the link provided previously, (Startup with Command Option R), then hit Reinstall Mac OS, which should install Catalina, (Note if no drive shows up, you may have to first go to Disk Utility while in recovery mode, select Macintosh HD (Usually) > File > Mount, (Or Unlock), then enter your admin password and try the reinstall again,
If all else fails, there could be corruption on the startup disk, which you can attempt to repair, but I’d try creating a bootable installer to do the job:
https://support.apple.com/en-us/HT201372
Hope one step works out! There are other options if not!