You can not write to an expression tag or a bound property on a memory tag. You will want to try a derived tag instead. Giving us more info and maybe we can provide more help.
I am trying to create a UDT instance with some of the tags inside the UDT being set to an initial value. If I cannot change the values later by writing to them that will not work.
I can easily create the UDT instance and then immediately write to the UDT changing the values to there initial state. I just thought it would be better to create the UDT all in one statement than it would be to have to create the tag and then write the values in a second statement.