completely re-install OS and to brand new

Or use these easy steps

1. Shut down the computer holding down the power button for 1,5-2 seconds or hold down the power button until you see the power menu click shut down and confirm or click the system menu in the menu bar (Apple menu) click shut down confirm and its shut down
2. start your mac holding the command+R keys to enter the recovery mode
3. in the recovery mode click reinstall mac os and confirm your password if prompted
4. agree to the terms and conditions
5. select the mac os drive
6. W A I T before the its done it can be up to 1h if you have clear space it can be lower to 10 minutes (You can see some restarts. Maybe make a cup of coffee.)
7. Setup the mac and done your mac is fresh and clear to sell it or start a new adventure on your mac

I hope you enjoy this quick tutorial

This is HTR. The mac expert from high sierra to big sur.

Thank you for the support.

Article for help How to reinstall macOS - Apple Support

Polish version of the article: Ponowna instalacja systemu macOS - Wsparcie Apple

And the video if the text was too long

Suggest getting the Full Installer of Catalina and use Safari as Others may not work. Safari will open a redirect and open the Apple Apps Store and choose the Catalina. It may alert you that Catalina is already installed - dismiss the alert and download it. Once it Downloads - it may attempt to launch. Quit it. Now there should be a new Application in Applications Folder called Install Catalina . What to Create a Bootable Installer and be sure to use a reliable USB Device with at least 16 GB space. Run the Create Media Terminal Script supplied in the link Create Bootable Installer to make that drive.

Now shutdown computer and disconnect all external drives except the USB Installer. Restart and hold the OPTION key and will be presented with option >> Choose the boot to the USB Installer. It too will present option and choose Disk Utilities >> View >> View ALL attached Drives. Chose the Upper Most Drive usually called Apple SSD and Not the Volumes indented and below. WE what to Erase the Total Drive and everything on the Drive - there is no going back. Erase the Drive- Give it a Name and Format as APFS with the GUID Partition Map.

One above is done - back-out of Disk Utilities and choose Install macOS and follow the prompts. The computer may automatically reboot several times in the process - Norma. It will eventually reboot and present the Setup Assist for Catalina.

Completely shut down your computer, then turn it on and when you hear the sound it makes when it comes on, hold the Command + R Key at the same time until the loading bar pops up. Wait and a screen should pop up, from there select "Disc utility" and "Continue" then you can wipe the HD.

Hello troyfipps,

Here are the steps to Reinstalling macOS Catalina and wipe all data: https://apple.co/2SO2LcO

If you are doing that because you are selling or giving away the Mac, you need to follow all of these steps as well:

https://support.apple.com/en-us/HT201065

Go here - Erase and reinstall macOS - Apple Support