You might receive QuickBooks Error 61 after running QuickBooks Desktop Install Diagnostic Tool or QuickBooks file doctor. The error will prevent you from using your QB Desktop software and hence you have to fix the error first. The QuickBooks error 61 occurs when you are trying to login into a company file in single-user mode but there’s already another user logged into it. The first thing that you should do to troubleshoot this error is to update your QuickBooks. If that doesn’t work, rename the ND and TLG files. This will repair any damaged data files and you can access your QuickBooks Desktop once again.