If you have the function DateTimeNow() in your custom expression or a text field with a calculated value, you may get the error indicating the expression is not valid for that context:
​Please specify an aggregation method.
Issue/Introduction
DateTimeNow not working without aggregation.
Resolution
To resolve the issue apply hotfix TS 7.8.0 HF-006 (TS-50469) and higher or TS 7.9.0 HF-001 (TS-50470) and higher.