My MacBook doesn’t have an admin account

It means the laptop was not properly prepared for sale. Even if a device is properly prepared for sale, I always recommend performing a clean install of macOS just to make sure it was done properly and to make sure the previous owner did not do something nefarious. In addition to making sure you have a factory clean OS, it also can alert you to possible issues with the device such as the device still being under control of the previous owner and whether a firmware password lock has been enabled......neither of these two issues can be fixed by the new owner (only the previous owner of the device can fix the first issue by releasing their management/control, while only the owner can fix the second).

Here is an Apple article of what the previous owner should have done prior to selling the Mac:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

As the new owner, you can only start at the Erase step of those instructions.

My guess is that you bought a used Mac, or received a hand-me-down that was set up by someone else, who forgot to make your UserID an Administrator, before they deleted their Admin account.

My best advice is to do a full TM backup of your Mac and then wipe the disk and re-install macOS from a USB installer and create a new (temporary) Admin account. (Use Google to find out how to create the Installer)

Then import your UserId files and the currently installed programs, etc. via Migration Assistant.

After that, use the temporary Admin account to promote your UserID to also be an Administrator.

You can then Log in as yourself and delete the temporary Admin, or else keep it around as a backup user.

If there were no administrator, no one could set up new user accounts on your Mac…so what user are you using?

Learn how to ⇢ Change Users & Groups settings on Mac - Apple Support

-Zachyy ✌️