Note: This procedure may require administrative rights.

In Windows Explorer browse to the folder specified in the error message. Delete all of the files within the final folder - except  for DBGrid.cfg index.mb and Index.db files.

After deleting the files go back into 2020DOC then highlight the table, and click -->Modify DB Structure-->mark as Indexed the fields that should be indexed then click -->OK. This will restore the table back to a normal state.