HiI get the PTH DES not avaiable. Or something like that.What does it mean?

I wondered the same thing. What I believe causes it is an incomplete route in the FMC, or possibly- you are descending AFTER the calculated T/D. ATC will sometimes not tell you to start descending until after the FMC calculated it, and it cannot follow the vertical path it wanted to since it would require too much of a dive. If you first got the message MCP ALT or similiar that means you didn't enter an ALT for the autopilot to descend to.