✓ Accepted answer
The problem is funny, actually. Considering you're trying to manipulate a BoolValue. It makes the error difficult to decipher.
Since 'button' is a TextButton, and 'Selected' is actually a property of TextButtons.. when you define 'isSelected' on line 2 the script thinks you're defining a variable based off of the Property's value.
So.. when you try to index 'isSelected' on line 4, the script essentially thinks you're trying to do "true.Value = false".
To fix this you're gonna have to simply change the name of the 'Selected' object.
button = script.Parent
isSelected = button.isSelected
isSelected.Value = false