Do I have to re install MACOS Big Sur when I erase my ID

The companion article linked above, has add'l instruction details that apply.

Also note #7:

The best way to restore your Mac to factory settings

is to erase your hard drive and reinstall macOS. 

"After macOS installation is complete, the Mac restarts to Setup Assistant

that asks you to choose a country or region. To leave the Mac in an out-

of-box state, don't continue setup. ~ Instead, press Command-Q to shut

down the Mac. When new owner turns on the Mac, the Setup Assistant

guides them through the setup process."

Depending on the initial or most recent macOS, you may be able to

leave Big Sur as it's OS. ~ While options usually include an older one

which the Mac had installed when new. A new owner may upgrade.

In order to be sure that everything is done correctly, please follow the steps here (all of them):

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

After the OS is downloaded, do not proceed with the installer - simply press the power button to turn it off. That way, the buyer can set it up using their own Apple ID.

The macOS installation should absolutely be completed before handing off to the new owner. However, when the Setup Assistant appears (after the installation), press Command-Q to shut down the Mac so that the new owner can set it up.

If you’re referring to registering the macOS installer itself with an Apple ID (to download it from the App Store servers), that system has been removed for most recent versions of recoveryOS (AFAIK). Older versions of OS X Recovery might still use that system though.

Icecoldkate wrote:

I am selling my MAC and have re set and it now is asking to re install MacOS Big Sur, is this correct?

You can not install and older macOS than what originally shipped with the Mac when new.

If it shipped with Big Sur, than that is it—yes

Otherwise you have options—

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

reference:

What to do before you sell, give away, or trade in your Mac ...

Icecoldkate wrote:

I am selling my MAC and have re set and it now is asking to re install MacOS Big Sur, is this correct?

Yes.

When you erase your Mac, you are erasing both your data and macOS itself. Once that's done, you should reinstall macOS (Big Sur).

Icecoldkate Said:

"I am selling my MAC and have re set and itnow is asking me to re install MacOS Big Sur, is this correct?“

———-

Important:

Sign Out & Disssociate

Have you signed out of iCloud on it, and disassociated it from your account(through Music, TV, or iTunes)? If not then do that. Elsewise, the buyer will not be able to associate it to their own account.

What was it in my post to the OP that needed correction?

FWIW, I posted a link to Apple's current instructions on what to do before selling a Mac and adding that they should turn it off before the setup begins.