I tried to explain this in a previous thread but could not make it clear, I think.
I found now exactly what went wrong, and maybe it can be fixed. Maybe not, and I should start from scratch.
Here is what it says in the Debian Handbook: https://debian-handbook.info/browse/stable/sect.installation-steps.html#id-22.214.171.124
4.2.9. Administrator Password
The super-user root account, reserved for the machine's administrator, is automatically created during installation; this is why a password is requested. The installer also asks for a confirmation of the password to prevent any input error which would later be difficult to amend.
4.2.10. Creating the First User
Debian also imposes the creation of a standard user account so that the administrator doesn't get into the bad habit of working as root. The precautionary principle essentially means that each task is performed with the minimum required rights, in order to limit the damage caused by human error. This is why the installer will ask for the complete name of this first user, their username, and their password (twice, to prevent the risk of erroneous input).
I do not, to my knowledge, have a super-user root account. If I do, it's the same as the one I use at login.
Can someone let me know what, if anything, I can do about this?