There are two reasons for those Zero stiffness messages--
1. You have declared all the members in your model as truss member using the STAAD TRUSS command. You correctly specified the partial member release command but as all the members are specified as truss, it is not effective. You need to change the first line of the model from STAAD TRUSS to STAAD SPACE.
2. You have assigned pin support at two nodes. Now consider LOAD 3 and 4 where lateral load in Global X direction is applied. There is no restrain in the lateral direction for the pin support. So there will be huge deflection in the lateral direction. If you restrain the lateral movement using one fixed support and stabilize the structure, there will not be any messages. The modified model is attached.(Please visit the site to view this file)