There are syntax errors in your query:
SELECT
CASE
WHEN a.Column LIKE '%Moderate to severe aortic stenosis%' THEN 'Moderate to Severe'
WHEN a.Column LIKE '%Severe aortic stenosis with%' THEN 'Severe'
WHEN a.Column LIKE '%moderate aortic stenosis present%' THEN 'Moderate' else ' '
END
FROM
MyTable a
Field a.Column
should appear after every WHEN
in comparison with LIKE
.