ERROR 000539: File "<expression>", line 1, SyntaxError: invalid token

Error Message

When creating a model with the ModelBuilder function in ArcGIS Pro using the Calculate Field tool and the output value of the Iterate Feature Selection iterator connected as an expression, the following error message is returned.

Error:   
ERROR 000539: File "<expression>", line 1 SyntaxError: invalid token

Cause

The error occurs because the output value of the iterator is used as an expression rather than an inline variable in the Calculate Field tool.

The tool has two outputs: Selected Features and group Value of the field for selected features, which could be used as an inline variable in other tools. Refer to ArcGIS Pro: Inline variable substitution for more information.

Solution or Workaround

To resolve the issue, define the Value of the Iterate Feature Selection as a Precondition of the Calculate Field tool and set it as an inline variable for calculating the field.

1. Remove the connection between Value and Calculate Field.

2. Make a new connection by dragging the arrow from Value to Calculate Field.
3. Select Precondition. Notice the connected arrow is dotted.

4. On the ribbon, in the ModelBuilder tab, click Run.

The image below shows the ModelBuilder function runs successfully.