In your microflow you need to use a “Change object” action. In there select the Member “Schritt” of $Montagueftrag and set the value to
$Montageauftrag/Schritt +1
You will need to decide if you want to commit this or not as part of the Change Object action.
Looks like arithmetic operations aren’t supported in Studio. I recommend you open the project in Studio Pro. The expression suggested by Robert is supported there.