WARNING!!! Make sure you type this precisely as the "rm -rf" means remove (delete) and the -r means recursive (all sub-folder items) and the -f means FORCE (it won't ask you to confirm). If this were entered improperly you could delete things permanently by accident. The command provided is correct and it should delete the "Install macOS Mojave" from the hidden trash folder in your home folder.
Reboot the Mac then Open Terminal and type:
sudo rm -rf ~/.Trash/Install\ macOS\ Mojave.app/
Type your password when prompted.
If you still need to downgrade to Mojave it is possible but it is a massive procedure to accomplish:
- Backup your Mac (Time Machine or Carbon Copy Cloner / SuperDuper!)
- Obtain a 16GB+ USB flash drive (probably need a USB-C to USB-A adapter)
- Re-download Mojave (see below)
- Format the USB flash drive to FAT32 or HFS+ and name it MyVolume
- Create a bootable macOS Mojave installer on the flash drive (see below)
- Reboot while holding the Option key and choose the flash drive booting into the installer
- Completely delete the internal disk with Disk Utility
- Re-install macOS Mojave and make sure you fully update it
- Copy your data back into place via the backup
Create a bootable USB macOS Installation Disk
https://support.apple.com/en-us/HT201372
Download an older macOS version such as Mojave
Open Terminal and type:
softwareupdate --fetch-full-installer --full-installer-version 10.14.6
Depending on the condition of your backup, you might have some issues. If you ran Time Machine after Big Sur then you may have trouble restoring on Mojave. If the Time Machine backup is from Mojave it should just work. If you run into trouble restoring you will have to manually copy your files from the Time Machine backup to Documents, Desktop, etc.
