Transmit Boundary condition is same as Flow out boundary condition and it is used for Lagrange Solver. The use of this B.Cs is that it will allow normal component of stress waves which reaches to this boundary condition to go out of domain. When you set up the transmit BC, it does state transmit (Lagrange), so when you apply it to Euler solver, you will get the warning, if you apply it to an Euler solver, no warning would be displayed.