I have followed a tutorial that suggested
1. Make sure your System Definition in SMBIOS is correct. If not, use Clover Configurator to generate the proper serial number2. Quit all applications, open up the Terminal and type:
sudo rm -rf /Library/Updates/*
sudo rm -rf "/macOS Install Data"
(this will get rid of any partial/incomplete updates you may have stored on your system while attempting to update)3. Instead of going the usual App Store route, run the following command inside the terminal:
sudo softwareupdate --dump-state -i -a
and update went through