Agree with @Luis Sequeira1- need to erase the Entire Drive. The method I am suggesting is going to be different, take some time but will result in the same End Result. Which ever method you are comfortable with is best for you.
How to Create a Bootable Installer. This will require a Stable and Reliable Internet Connection much like using the Internet Recovery Mode. This computer probable has the Touch ID / Secure Enclave Chip and therefore, by default, will not allow booting from an External Drive. That can be overcome but reading this About Startup Security Utility. This step MUST BE DONE before attempting anything else. Once that is achieved, procceed to make the Bootable Installer.
Now shutdown computer and disconnect all external drives and connect to charger. Attach the Bootable Installer and restart the computer. Immediately Hold the - Option - key. This will load the Boot Manager and present to boot from Internal Drive or Bootable Installer >> choose the Bootable Installer. It too will offer options - Choose Disk Utilities ?? View >> View ALL Attached Devises. Choose the Upper Most Drive ( no Volumes listed below it ). The Drive should have the name Apple Media or maybe Apple SSD. That is the Drive we want to work on. Choose that Drive >> Erase and give it a name like Macintosh HD >> below that Format as APFS Format and below that Scheme: GUID Partition Map >> Erase. That Done Now, Backup Out of Disk Utilities and Choose Install macOS. This will start the Installation Process and may Automatically Reboot several time - Normal. Follow the prompts util - if all goes well - will do a final reboot and present the Setup Assist.
