@ExtendWith(MockitoExtension.class)
class BoardTest {
    @Mock
    Piece piece;

    private Board instance;

    @BeforeEach
    void setUp() {
        instance = new Board(piece, 10);
    }

    @Test
    void getCostPerSpace() {
        when(piece.getPiece()).thenReturn(PieceType.SQUARE);
        double expected = 2d;
        assertThat(instance.getCostPerSpace()).isEqualTo(expected);
    }
}

@ExtendWith(MockitoExtension.class)
class BoardTest {
    @Mock(answer = RETURNS_DEEP_STUBS)
    Piece piece;

    private Board instance;

    @BeforeEach
    void setUp() {
        instance = new Board(piece, 10);
    }

    @Test
    void getCostPerSpace() {
        when(piece.getPiece().getCostPerPieceSpace()).thenReturn(2.5d);
        double expected = 2d;
        assertThat(instance.getCostPerSpace()).isEqualTo(expected);
    }
}

You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.

Underlying exception : org.mockito.exceptions.base.MockitoException: Unsupported settings with this type 'com.senorpez.game.PieceType'
org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class com.senorpez.game.PieceType

If you're not sure why you're getting this error, please open an issue on GitHub.