There are several possible causes for this issue.
Related to a bad merge field or mismatching TableStart and TableEnd merge fields in a detail region. You might also see an operation was cancelled error message.
WHAT TO DO
A good start is to remove paragraphs (or entire pages for larger templates) and repeat the merge until you find the issue. Then you can correct the merge field or detail region causing the problem.
For example, you may receive this error message if the TableStart and TableEnd merge fields are not in the same row of the detail region. The correct usage of TableStart and TableEnd merge fields is shown below.
An IF statement in a Microsoft Word template with data being merged into that IF statement containing double quotes.