Its hard to debug a picture! Especially a pic where you don't show the definition of all variables in use!
Always better to send the worksheet!
Try typing Tb(n2) instead of just Tb when you define your function X
OK, two problems:
The first one is as already said in my first answer that you defined Tb as a function and therefore you have to use it like a function and not like a variable.
The second problem is the range (0 to 2) you are looking for a zero of function X.
In this range X has two zeros and a pole! The pole at approx n2=0.976087 is the reason that "root" would fail.
Solution is to use "root" twice with smaller intervals (which can be seen in the plot). You have to decide which of the two zeros is the one appropriate for your problem.
Worksheet in format Prime 5 attached