Free Size optimisation

Hi, 

I am running a free size analysis on a 1800 mm DIA tube using a static linear load step with a GRAV load card as it is a self weight problem. The tube is simply supported and 33 000 mm in length. 

When I create a displacement constraint the constraint cant be created as the load step is not compatible with the design variable, see below. 

The model is a PSHELL surface model. 

I am attempting to minimize the volume fraction of the tube to reduce weight based on the displacement. The design variable is also using a zone based approach to keep section at the same thickness and the design variable has a stress constraint on the DESVAR card. 

How can this static load case not be accepted by a simple dconstraint set up? 

Thank you