To solve this error, complete the following steps:
1) Open your QuickBooks and log into your company QuickBooks file.
2) Then, click on 'Edit' at the top of the screen. From there, click on 'Preferences'. Then, go to 'Integrated Applications'.
3) After this, click the 'Company Preferences' tab. Then, click on 'LMN QuickBooks Sync' and click the 'Remove' button.
4) After doing so, go back to your LMN QuickBooks Sync Tool and re-complete Step 2. Connect to your QuickBooks company file.
5) QuickBooks will now show a screen which asks you to give permission to the LMN QuickBooks Sync Tool to access your company data.
After you accept the access request, you will have successfully connected your LMN QuickBooks Sync Tool to your QuickBooks company file.