The frontend application marks errors by drawing a red ring around the offending node and offers a brief description of the error on-screen.
When running a trading session, the session node is usually the one featuring the error notice.
When working on a strategy, errors in the syntax or logic of conditions and formulas are displayed similarly. In such a context, make sure you turn on the Formulas & Conditions layer of the corresponding session and look around within your workspace. Hover the mouse pointer over the node with a red ring and you should get a description of what is that is causing the unexpected behavior.
Bear in mind that when the slider is fully closed, errors occurring in a simulation will no longer show up in the design space, as shown in the below video: