How do I erase a MacBook that was removed before the process?

Removing the Mac from Find My without erasing it first can leave the buyer with continued access to whatever was still signed in on that Mac (iCloud content, iMessage, FaceTime, synced data, etc.). The good news is: you can still lock things down quickly, even if you can’t directly wipe the Mac anymore through Find My. The priority now is to secure your Apple Account and remove access, then (if possible) erase the Mac.

Here’s what you should do immediately, in order:

• Change your Apple Account's password ASAP. This is the fastest way to stop any continued iCloud access from that Mac.
• Sign out that Mac from your Apple Account (even if it’s already removed from Find My). Even if it’s not showing in Find My, it may still be listed as a trusted device.
  • Go to appleid.apple.com
  • Scroll to Devices
  • Click the MacBook (if it’s listed)
  • Choose Remove from account
• Review and remove any unknown trusted devices. If the buyer had access long enough, you want to confirm nothing else got added.
  • Still on appleid.apple.com → Devices
  • Remove anything you don’t recognize
  • Also check: appleid.apple.com → Sign-In and Security. Look for any unexpected phone numbers or trusted info
• Turn on 2FA (or verify it’s enabled). If they didn’t already have it enabled, this is critical.
  • Settings (iPhone/iPad) → Apple ID → Password & Security
  • Confirm Two-Factor Authentication = ON
• Secure iMessage and FaceTime access. Especially important if Messages were still enabled on the Mac.
  • On iPhone: Settings → Messages → Send & Receive
  • Confirm only your numbers/emails are listed
  • Do the same under Settings → FaceTime
• If there’s any chance they had browser access: change key passwords
• If they were still logged into the Mac and had access to Safari autofill or saved sessions, I’d strongly recommend changing:
  • Email password
  • Banking/financial passwords
  • Google/Microsoft password
  • Social media passwords

If the Mac was already removed from Find My, you likely can’t send a remote erase command anymore. The only way it gets wiped now is:

• The buyer erases it themselves (recommended), or
• You regain access to it long enough to erase it properly

If you can contact the person who has possession of the Mac, this is what you should tell them. Of course, I'm assuming that you sold it and that it was not stolen. Otherwise, ignore the following.

“Please erase the Mac completely by going to System Settings → General → Transfer or Reset → Erase All Content and Settings. This removes all my information properly and lets you set it up cleanly.”

If it’s an older macOS version where that option isn’t available, they can erase via Recovery Mode, but usually the built-in erase option is easiest.

If you have already given the laptop to someone else without preparing it properly first, then there is little you can do to erase or otherwise secure the stuff that was on it short of getting the computer back.

FYI, this is what you should have done: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

The fact that they have now asked you to remove the computer from your Apple Account and Find My suggests that they are trying to set up the computer for themselves. That release will allow them to erase the drive and reinstall a fresh copy of macOS.

Unfortunately, there is no guarantee that they will not go snooping into whatever data you may have left behind. On the other hand, it may be that they simply are honestly just trying to make the computer their own.

To be safe and maybe even overly cautious - or not - you should consider changing the password of your Apple Account and any others you are concerned might be compromised in the hands of the Mac's new owner.

Sorry, but what exactly are you trying to do? What do you mean by "I removed my macbook" before erasing it?

Are you trying to prepare this Mac so that you can sell it or trade it in, or trying to update/upgrade its operating system, or something entirely else?

We can only go by what you provide us.

Ref: Writing an effective Apple Support Commun… - Apple Community