The Query Builder in ArcGIS Pro 2.1.2 returns the error, "The SQL Expression has invalid syntax" when adding a definition query to a shapefile using a field named, 'Group'.

Workaround

1. Add another field called, 'Groups' and calculate all values into it.
2. Import the shapefile into a file geodatabase.
3. Manually enter the SQL query in the SQL window instead of using the Query Builder.

Steps to Reproduce