Cannot use expression variables in pipeline

I have set an expression variable for image tag as [document ](https://docs.gitness.com/reference/pipelines/expression_variables#buildbranch) described, but an error "**unable to parse variable name**" when I run the pipeline.
The pipeline yaml:
![image](https://github.com/harness/gitness/assets/4284380/874bd07b-8180-473b-b897-6015ae78524c)
