I needed to create a global variable and use an expression to set its value. However, when I choose expression and enter an expression the global variable returns (not set).
Webcore Global Variable Expression
You can use the evaluation console to calculate the result of an expression, then store that value as a global variable. If you need it to update, use a timer in a piston to update the global variable value as needed. Otherwise, global variables do not support being defined as expressions, just static values.
Is this a cross-post from the Hubitat community?
When you say you enter the expression, are you setting the variable within your piston or at the start of the execution? it would help if you posted a green screenshot of your whole piston to see what you might be doing wrong.
Also, your global variable will most likely not be an expression but a value. Then you would use an expression to set that variable to a particular value when you want to update it. Setting a global variable to an expression does not mean that it will update every time the component of the expression is updated.