In my model I get this error message :
The allowable stresses are not specified properly , the required pipe thickness ca not be computed.

in my model i have two materials and I am using ASME B31.8 VIII as i specified in the configuration file but i got this message and for the X65 material I see no calculations done or code check for it.just code check is done for the super duplex material only.
how can i solve this? Is there a bug when using ASME B31.8 VIII?

You should be seeing allowable stress values. For B31.8 VIII the allowable is just a constant times Sy.

The issue with the minimum wall thickness is that B31.8 VIII doesn't need the longitudinal joint factor ("eff") for the allowable stress, so it is grayed out in the input. Unfortunately this value is needed for the minimum wall thickness calculation.

We'll have to change the status of this "edit box" in the Input Processor so that you can define joint factor for B31.8 VIII.