Steve’s suggestion is good.
But to directly respond to why your original expression didn’t work; it is exactly as the error message says, the if-result is not the same type and the else-result.
A valid_if expression must return a TRUE or FALSE (yes/no) value. Your else-result, ISBLANK() does that. But your if-result , [_THIS], is returning the value in the column (a datetime type).
If you had replaced [_THIS] with TRUE in your original expression, I believe it would have worked.
Hope that helps.