I've just bought an Acer Nitro 5 AN515-57-76WE laptop (with no OS), and its hard drive came with some partitions. However, when I installed Ubuntu, I chose the erase disk, and install option. Later on, I read on a link (I can't find it anymore), that I should have not formated the UEFI partition... No, I'm afraid I have done something really bad.
When I start the laptop, I get several error messages, then the main screen with the Acer's logo, and Ubuntu's just below.
Every now and then, when I'm working on Ubuntu, the laptop halts (at least graphically) for a few seconds...
Was this my doing? What problems may I get in the future with the laptop, without an UEFI?
You did everything correctly. Ubuntu installer should have created an EFI partition for you.
The advise to keep an EFI partition is related to a dual boot installation.
So there is nothing to worry about in this regard.
If you have some problems with your system, you can always ask a new question.
A first guess would be that you need to install graphics drivers. But this needs more details.