Al has a 2015 Macbook and he's running out of space, so he can't install Big Sur. Leo says that Al should get an external drive to copy off his data to free up space. Maybe even make a bootable copy of the hard drive and then start from scratch with Big Sur after erasing the drive. Then restoring the data. So get a terabyte external drive and try that. Time Machine is also a good option with that external drive. That's Apple's own way.
But is Big Sur worth it? Not for an older Mac. So if he's up to Catalina, that's good enough for that 2015 Mac.