How do I lock my stolen Mac mini when I don’t have the receipt?

What do you mean your log in on iCloud is closed?

No need for a receipt, it will not help you will a lost device. The only thing that will is FindMy and that would need to be turned on. Follow the steps here in all cases:

If your Mac is lost or stolen - Apple Support

"MY OPEN MACMINI WAS STOLEN,HOW SHOULD I CLOSE IT DOWN?: MY MAC MINI WAS STOLEN WITH THE PINCODE WRITEN ON IT,HOW SHOULD I CLOSE DOWN THE MAC AND I BOUGHT IT ON APPLE STORE BUT CANT FIND THE RESIEPE MY LOG IN ON ICLOUD IS CLOSED,THERE ARE THERECIEPT"

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Don't Use All Caps:

Keep your posts out of all caps. It makes you posts hard to reach and decypher.

Retrieving a Stolen Mac:

I.  Locate This: Locate a Lost or Stolen Device - Apple Support

II.  Getting this Back:

• Backtrack your Locations - If at a coffee shop, stop in, and ask it if was placed aside.
• Check with Local Authorities - Perhaps it was found and was turned in.

III. Being Stolen, Modify your Credentials:

• Modify: your login credentials (Using Both a New eMail and a New Passwordfor each and all sites)
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID and for all other accounts that have such an applicable option

Even if Find My was enabled and the thieves do not know the Apple ID password, if they know the local administrator password, the thieves can ransack any data on the Mac and may be able to shut off Find My too.

"How do I lock my stolen Mac mini when I don’t have the receipt?"

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LoJack:

Did you have any third-party forswore on it for locking and deleting the device? LoJack would allow it to be locked, but the thief would have to be logged into your Apple Account for it to be seen.