Count unique orderers

You can create the relationship in the Model pane of in Power BI (see image).

If you are unable to create the relationship, then your measure will need to use the LOOKUPVALUE. Try this:

Ordering Sites = 
    COUNTROWS(
        SUMMARIZE(
            ADDCOLUMNS(
                'Sales',
                "SiteID", LOOKUPVALUE(Terminal[SiteID], Terminal[TerminalID], Sales[TerminalID])
            ),
            [SiteID]
        )
    )