From the screenshot you uploaded, i find that you didn’t create the variable, just you wrote the word screenshot. If you want to Create the variable there are two ways.
In properties of save image activity - Output - Screenshot - Press Ctrl+K, It will prompt for the Variable input there place your variable name.
Directly got to variables which you can see below of the workflow window, there you will find create variable, click that, name your variable and select the type where directly image will not display, you need to select the browse types and select the image from the list by typing UiPath.Core.image.