FILTER only positive values

Suppose the lowest level of your hierarchy in your visual is Table1[Item]. Then you need to iterate over each of those items.

Availability ( hours + ) =
SUMX (
    VALUES ( Table1[Item] ),
    VAR __available = [Target Capacity] - [Planned Hours ( Total )]
    RETURN
        IF ( __available > 0, __available )
)

You can use other columns from related tables. See the documentation.

It might work better to start with Skills and use Staff[Name] though.

Availability =
VAR Summary =
    SUMMARIZE (
        Skills,
        Staff[Name],
        Skills[Skill],
        Skills[Experience level],
        "@Hours", [Target Capacity] - [Planned Hours ( Total )]
    )
RETURN
    SUMX ( FILTER ( Summary, [@Hours] > 0 ), [@Hours] )