Understanding the Error Message:
The error message "Error syncing invoice: Error adding item "Item Name": BadRequest. Details: The name supplied already exists. : Id=2641 ," suggests that there's an issue with adding an item in QuickBooks Online. It specifically indicates that a name supplied already exists within the system. This could be a name clash with an employee's name, an existing item name, or any other entity within QuickBooks Online.
The Cause of the Error:
The error typically occurs when ServiceBox tries to add a new item with a name that contains a separator, which, in this case, is a space followed by a dash and then another space (e.g., "Manpreet - supervisor "). This separator is used in most cases by ServiceBox to split the item name and description in QuickBooks Online when creating a new line item. However, if this separator is used in a way that clashes with existing names or items, it can trigger the "Error-Adding Item" message.
Resolving the Issue
1. Change the Separator: One effective solution is to replace the current separator (space-dash-space) with a different separator that doesn't clash with existing names. This could be an underscore (_), a comma (,), a colon (:) or any other character that isn't commonly used in your naming conventions.
2. Modify QuickBooks Online Settings in ServiceBox: Click on the user name appearing on the top right corner > go to 'QuickBooks Online' > Export/Import settings> If the space-dash-space separator is used here, consider updating it to the new separator you've chosen. This ensures that the system recognizes the correct separator for splitting item names and descriptions.
3. Adjust Naming Conventions on the line items : If you've been using the space-dash-space separator in ServiceBox QuickBooks settings as shown in the image above.
Then, you may consider changing the separator used in the line item to a different separator or set up the line item without the spaces if you still want to use dash as a separator.
To edit the line item in ServiceBox> please go to settings> Quote Items> Edit the line item > click Save. 
4. Avoid Duplicate Names: Always double-check for existing names before adding a new item. QuickBooks Online requires unique names for different entities. By avoiding duplicate names, you reduce the chances of encountering the "Error-Adding Item" message.
5. Special Character's : If any line item has colon or semi colon, QuickBooks Online treats it as special character, so user might need to replace this with another symbol.
Please feel free to reach out to support@jobboxsoft.com, if you have any further questions, will be happy to help.