Hello, @Ahmadzai. We understand how hard it can be when a feature you rely on is not functioning as expected, I'm here to share insight about your experience.

If you're getting error codes -6190 and -816 when another user is logged into the company file in single-user mode.

To resolve this, ensure that your QuickBooks release is up-to-date, then rename the ND and TLG files to repair the company data files. This could be due to a mismatch between the transaction log file and the company file, causing the error when trying to access it in single-user mode.

Here's how:

1. Open the folder that contains your company file.
2. Find the files with the same file name as your company files but with extensions.ND and.TLG. 
      company_file.qbw.nd
      company_file.qbw.tlg
3. Right-click each file and select Rename. Add the word OLD at the end of each file name. Ex: company_file.qbw.nd.OLD
4. Open QuickBooks again and try to sign in to your company file. If the same issue occurs, proceed to solution 
   
   

You can also check out this article for more troubleshooting steps about the error -6000: Resolve Error -6000 when opening a company file. 

If the issue persists, let's create a back up copy of your company file. Then, download and install QuickBooks Tools Hub, then run the Quick Fix my Program. This is a built-in tool that auto-detects and auto-fix data damage issues within QuickBooks.

1. Download the most recent version (1.4.0.0) of the QuickBooks Tool Hub.
2. Locate the file you downloaded (QuickBooksToolHub.exe).
3. Follow the on-screen steps to install and agree to the terms and conditions.
4. From the tool hub, click Company File Issues.
5. Choose Quick Fix my File.
6. Click OK when it completes and launch your QuickBooks.
   
   

You can check out this article for more troubleshooting steps about the error when changing the company file name: Resolve Error -6000, when opening a company file.

You might also consider reading this handy article discussing access limits in single or multi-user mode: Tasks that can be performed in a single or multi-user mode in QuickBooks Desktop. 

Should you need more help managing your company data, please don't hesitate to revisit this thread. I'll be right here to keep assisting, @Ahmadzai. Have a great day ahead!