You are declaring your Text widget as a const, which requires all of its children to be const as well. If you want to fix this, you should not use a const Text widget in this case as you want to pass a non-const variable.
The reason for this is that Flutter uses the const keyword as an idicator for a widget to never rebuild as it will get evaluated at compile time and only once. Hence, every part of it has to be constant as well.
double textSize = 10.04;
// ...
child: Text('Calculate Client Fees', style: TextStyle(fontSize: textSize))
Read more about it in this article.