您需要用表别名限定matricula以使其明确。

JOIN (
    SELECT e.matricula, SUM(total)/(kmsTotais) AS custoKM
    FROM (
        SELECT matricula, SUM(kmfim - quilometragem) AS kmsTotais
        FROM formulario
        GROUP BY matricula
    ) e
    JOIN (
        SELECT matricula, abastecimento_euros as total FROM formulario
        UNION ALL
        SELECT matricula, custo as total FROM viaverde
        UNION ALL
        SELECT matricula, valor as total FROM reparacoes
    ) as subquery ON e.matricula = subquery.matricula
    GROUP BY e.matricula
) i ON i.matricula = f.matricula

您也可以将ON e.matricula = subquery.matricula替换为USING (matricula)，因为这表示两个表中的列名相同，所以不需要限定该列。