Summary
Attributes, Associations and Operations cannot simultaneously be abstract and class.
Details
The 'class' status of an Attribute, Association, or Operation makes them static, meaning only one instance can exist at any given time.
The 'abstract' status, by definition, stipulates that an element cannot be instantiated. The two properties are therefore incompatible and cannot both be true at the same time.
Tips
Choose between these two statuses, and set only one of them to true.