Well, .ND and .TLG are both configuration files that allow you to run QuickBooks company files on the network and in multi-user mode. If there’s any damage in such files, then it might lead to QuickBooks Company File Error 6155, and hence, you are required to rename it to eradicate the error from its root. For this, carry out the steps enumerated below:
In the beginning, open your QuickBooks company file.
Now, navigate to the following path:
C:\Users\Public\Documents\Intuit\QuickBooks\Company File [Default Location]
Look for the files having .ND and .TLG extensions.
After this, hit right-click, then select Rename option and also type .OLD at last in the file name.
The final step is to tap on Yes to confirm the action.

Source - https://dataservicesolutions.com/blog/resolve-quickbooks-error-6155-unable-to-open-company-file/