Although the variable is present in the Variable Set, you are not able to select it for a particular preflight check.
When configuring a preflight check, you must first select the Variable Set concerned. Only variables that belong to this set can be applied.
Variables are (among others) defined using a particular type (Constant, Rule Based, Job Ticket) and value/measurement type (Number, Length, String, Boolean). This value type must match the preflight check settings it is intended for, otherwise it is not displayed when you try to select it.
Make sure the correct Variable Set is selected. If the variable you want to use is present in a different Variable Set, you may decide to switch Variable Sets or add the variable to the currently selected Variable Set.
Next, review the variable and ensure it's defined as the right "value type" (Number, Length, String or Boolean) to match the preflight check.