It's hard to know exactly what it means, but in my newest computer (a Toshiba), there is a clear entry 'boot mode' where I can switch between UEFI and CSM mode.
If your computer can boot Ubuntu in UEFI mode, maybe you should stay there, and just clear the partitions. Now that oldfred has found this thread, you will get the best available support, so you will manage to get a good system, whichever boot mode you choose
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