So, you are getting an error because you are trying to call the DataCard as a Fieldname for the Gallery Items. You need to use the name of the field that contains the values "New Train", "Add Loco", "PAP/PDP", etc.
Easiest was to figure out what that field name is would be to select Checkbox1, and check the "Text" Property value. It most likely says something like:
ThisItem.Value
If so, replace ChangeType_DataCard3 in what you have above with the whole bit that comes after:
ThisItem.
Meaning if the Text property of Checkbox1 is indeed
ThisItem.Value
then you should have the use the following for the Required Property:
First(
Filter(
Gallery1.AllItems,
Value = "New Train"
)
).Checkbox1.Value <> true
If it is not 'Value', and you are still having some trouble, would you post a screenshot showing what you have entered in for the Items property of Gallery1?