QuickBooks multi-user mode not working is an issue that can occur when multiple users try to access a QuickBooks company file simultaneously. The issue can be caused by incorrect network configurations, damaged QuickBooks installation, or an outdated server operating system. Checking network connectivity, verifying firewall and antivirus settings, restarting the QuickBooks Database Server Manager, and updating the server operating system. It's also recommended to seek assistance from QuickBooks technical support or consult online forums for additional troubleshooting solutions or dial 1.855.738.2784 to connect with a QB Professional.