The Complete Message:
Carrier system type A15 (in X / Y direction) is selected. Curtains should be used in A15 carrier systems.
The A15 carrier system is defined in the earthquake code as buildings where all the effects of earthquakes are met with high ductility level reinforced concrete walls.
However, the curtain is not defined in the project.
Possible solutions:
Curtains should be added in the requested direction in accordance with the A13 carrier system in the project .
If a curtain will not be used in the structure, in the Analysis Settings / TBDY 2018 Earthquake Code Wizard, A11 for High Ductility Level Systems and A31 for Systems with Limited Ductility Level can be selected.