@PhD_Wallyson In this latest worksheet, you handle the symbolic coefficients correctly. That's good. Continue doing it that way.
The interface conditions, however, don't look correct. You have:
(D@@2)(X)(L) = -(kr)*(D(X)(L))+(D@@2)(X)(L), # equilibrium condition bending moment at x=L
A beam's bending moment is EI X''. Shoudn't the interface condition have EI in it?
One more thing. In the worksheet image that you posted today, we see that the μ values are calculated incorrectly. We even have a negative μ! I expect that once you correct the missing EI issue, that problem will go away. And then the plotting of the mode shape will work as before.
Yet another comment: The legnths L1, L2, L3 in the worksheet are in centimeters. Be sure that kt, kr, E, I, A, etc., are in compatible units.