You may see this error when you try to preview or activate your app (see image below). This error is caused by a blank or not well-formed logical expression:
An expression is created with an expression editor, and an expression editor is found in many places throughout your app including:
An expression is blank when it is empty. For example, if you click "conditionally display" on the footer of a page block but you don't write an expression.
An expression is not well-formed when it is only partially complete or when it is logically inconsistent. In this case, there will be a hazard icon next to the expression:
The error message provides details about where you can find the blank or not well-formed expression. Often it will give you the name of the variable that is being assigned (or "defined") but the blank or not well-formed expression.
For example, this error message is telling us to go find the expression that defined the variable called `full name`:
This depends on what exactly is wrong with your expression.
If the expression is blank (i.e. empty), you can either write a new expression or remove the expression entirely.
If the expression is not well-formed, add or remove elements from the expression until the hazard icon disappears.