The program was compiled with the following error,

! Undefined control sequence.
l.64 ...p(- \frac{E_j - E_i}{k_B T}) & \quad \text
                                                  {:  E_j > E_i}\\
?
! Undefined control sequence.
l.65     1 & \quad \text
                        {:  E_j<E_i}\\
?

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The main reason is that \text is not found and you just need to add the relevant package, for example, \ Text in this example needs to add ams Math Package.