I am getting that error when I download the app.
So somewhere in your app you are using the lesser than operator. One of its param (on the LHS) is an empty list. An empty list cannot be compared.
Please share your blocks for further help.
Is this the only place in your app where comparison operators are used?
Yes it is the only place
But then the error shows less than.. (<)
Show all blocks.
You are using the BLE .StringsReceived event block, which always returns a list.
When you get a list, you have to work through its items one by one, using a for each item in list block.
Apply the tests to item inside the for each loop.