QuickBooks error code 99001 occurs when you try to open a multi-user file in a single-user setup. The error message reads appears as: “QuickBooks is unable to open this company file”.
Download and run the QuickBooks File Doctor. This is important so please do this carefully. If this doesn’t help, move on to the next solution. But this one is the easiest as here the only thing you need to do is to install the file and try to run it.
If the company file is open in Single User mode in another workstation
Step 1: Exit QuickBooks Desktop in all devices.
Step 2: Open the company file on the Host device.
Step 3: Visit the File menu, then choose Switch to Multi-user mode.
Step 4: In case you are getting the same problem, reboot all workstations.
Also Read: How to Fix QuickBooks Error c=1303?
If the company file is open in Single User mode on the Host computer.
Step 1: Visit the File menu, then select Close company.
Step 2: Re-open the company file. Visit the File menu, then press Open or Restore Company.
Step 3: Choose Open a company file, then Next.
Step 4: Apply your company file.
Step 5: Press Open file in multi-user mode, then Open.
If your company file is located in a read-only network folder
Step 1: Visit the computer that is hosting your company file.
Step 2: Open the folder where your company file is saved.
Step 3: Right-click on the folder, then select Properties.
Step 4: Visit the Security tab.
Step 5: Choose the user that is having issues to log in, then Edit.
Step 6: Check Allow for Full Control.
Step 7: Press Apply, then OK.
In case the computer is being accessed remotely via LogMeIn, GoToMyPC, etc.
Step 1: Right-click on the taskbar, then press Task Manager.
Step 2: Visit the Processes tab. Check if there are running processes under the name of the user that you are having issues with.
Step 3: If there are processes under that user, restart your computer.