You can perform the search but you need parenthesis to group the 'NOT' part, e.g.
LIKE Schulze NOT (Schools OR School OR Scholz)
However, the 'NOT' is not attached to the variations of Schulze it's applying across the whole document. So if the document has the term 'Schools', 'School', or 'Scholz' then the document will be skipped.
It would appear that you want an 'EXCEPT' feature that would tie the following expression to the preceding one to limit the variations, this request is already in our Issue Tracker.
Also note that the term 'NOT' does not have a colon after it.