Power BI DAX Function 'SWITCH' does not support comparing values of type True/False

is this a colunm or a measure 

if measure then you must use a max of the colunm values that you are comparing 

Status = 
SWITCH (
    TRUE(),
    MAX([G1TDTE]) = "Null", "Plan",
    AND ( max([G1DDTE]) = "Null", max([G1CDTE]) = "Null" ), "Undone Wip",
    max([G1CDTE]) = "Null", "Done Wip",
    "Finished"
)

for colunm no max need use the col ref as is 

Status_COL = 
SWITCH (
    TRUE(),
    [G1TDTE] = "Null", "Plan",
    AND ([G1DDTE] = "Null", [G1CDTE] = "Null" ), "Undone Wip",
    [G1CDTE] = "Null", "Done Wip",
    "Finished"
)

this only works if you have replaced you colunm with text colunms