At first, yes, i used weird characters like θ τ β γ. But then i make the same thing but changing θ for "theta", τ for "tau" and so on, but in the end it gave the same error message
EDIT:
I have re-write all the formulas and now is working (I think the "weird character" was m^4, changed it for m4), but just when I use the predefine units, if I chage the unit, I get an error message