QuickBooks Multi-User Mode
QuickBooks must operate in single-user mode when performing a number of tasks including:
Creating Portable Company file
Rebuilding Data
Copy Company File
Importing & Exporting
Set Closing Date
1099 Wizard
Change Bank Feeds
Schedule Reports
Work with Accountant's Copy
Payroll Setup
Changing Company Preferences
Sometimes folks forget to revert to multi-user mode after performing single-user tasks. Click File, then "Switch to Multi-user Mode" or vice versa get yourself to where you need to be.

Slingshot Information Systems is well versed in supporting QuickBooks users. Reach out if you ever need assistance.