Make sure that the object you are trying to access in the expression editor is in scope/available in the microflow. If its not, then retrieve it before using in an expression.
Attributes and associations of object variables are accessed using a slash. For example, $Customer/Name, $Customer/Grade refer to the attributes Name and Grade of the entity Customer.
For more details, please see Microflow Expressions in the documentation.
One of the reasons can be the object is not available in the microflow.
If that's the case you should first retrieve this.
If this is not the case, a screenshot of the microflow and some more explaination would be appreciated.