This will solve your problems.
1. Boot to INTERNET recovery:
Command-OPTION-R
at boot
2. You'll need your wifi password, and be patient as the utilities load (spinning globe).
3. When you get to the utilities, open disk utility.
4. VERY IMPORTANT STEP:
Go to the "view" menu and choose "show all devices".
(If there is NO "view menu", skip to step 5)
5. Look at "the list on the left". The top item should be the physical hard drive. Click ONE time on it to select it.
6. Erase it to APFS, GUID partition format.
7. When done, quit disk utility and open the OS installer.
8. Start "clicking through". The Mac will restart one or more times, and the screen will go dark for a minute or two with no other indications of activity. BE PATIENT.
9. When the install is done, you'll see the initial setup screen "choose your language". If you're selling it, at this point either quit the installer (if a "quit" menu choice is offered), or... just hold down the power-on button continuously until the screen goes dark and the MBP powers off.
It's now ready to hand over to the new owner. The next time it boots, the new owner will see the "initial startup screen" again, and set up from there...
