The ntext data type cannot be selected as DISTINCT because it is not comparable.
There are certain text fields in this report that cause the problem. If you cast them as say nvarchar(500) then this solves the problem.
There's a calculated field in the header Tax Invoice DocNum which references the above. Remove this and replace it with a standard text field.
If it exists, remove the DOCUMENT PRINTED HISTORY table from the query. It adds no value.
If you receive this error in relation to your reports, download updated reports:
Alternatively, if you have made changes to the templates, you may need to adjust the data layout inside the reports.