Could the problem be that the sd card is corrupted? My windows 10 computer does sometimes say there are problems with it. So... I don't know. If not... can I create a new user?
Yes, it is possible (but far from certain) that the micro SD card is corrupted.
Writing a fresh version of the Raspbian Stretch Operating System to a fresh micro SD card as a comparison would check that possibility.
A Windows Operating System will almost always fail to recognise the linux-formatted root partition on the microSD card and advise, wrongly, that it needs to be formatted. That is a design decision on the part of Microsoft, not necessarily an indication that there is anything wrong with the card. Always
refuse the offer to format the 'drive' containing your Raspbian installation.
To be able to create a new Raspbian user won't you need to log in first... ?