Further to Jim's comment:
1) Date.ToText is only going to work if you have applied a Changed Type step and made sure the Date column
data type is Date
2) Your formula as Jim mentioned should look like this:
Power Query:
if ([Movement] = "opening" or [Movement] = "counting") then "" else Date.ToText([Date])
3) Although in Excel your only option is to insert "" in Power Query you could have used "null" (without the quotes).
This leaves the cell blank rather than having it filled with an empty string.
PS: Do you really want the resulting Date to be a Text String ? That is bound to cause you issues later on.