Please help me

Firstly, what storage did your Acer Nitro 5 AN515-12 come with? Did it come with the Intel SSD 600p SSDPEKKW256G7, 256 GB boot drive (which is very small and is the reason why you have "Disk 100 Percent by Systém").

Do you also have the WDC WD10SPZX (1 TB, 5400 rpm) as a slave drive on the Acer Nitro 5 AN515-12? As its impossible that "Disk 100 Percent by Systém" is taken up if you have a twin storage drive and you have put all your personal files on the 1TB WD slave drive.

Anyway, you must not have cleaned your boot (C:) drive SSD from the "Windows Update" backed up files and that is why this happens, especially on a small boot drive like a 256GB M.2 SSD which should be upgraded to at least 500GB or higher SSD M.2. Also, these files take up allot of extra space and you should do this maintenance of your SSD every time and after every "Windows Update". These updates take up allot of GB's of extra and unwanted space, also, inspect and delete some of your unwanted personal files.
To do a Disk Cleanup do this:
1. Go "This PC"
2. Right click on (C:) > Properties > Disk Cleanup > Clean up system files 
3. Delete most of those files and update windows files e.g. temp files and all the rest. For the "Downloads" make sure that you don't delete these files except if you have backed them up as these are all the files are the "Downloads" that you have on your computer.

If this doesn't free up allot of space? I would suggest that you upgrade your boot M.2 SSD to a bigger capacity drive e.g. 500Gb, 1TB or 2TB or whatever your budget is.