Hello Gerda Coeman 

Could you explain your processes in a little more detail please. Are you receiving a PPX package? or maybe something else?

When you receive a PPX from the client, this is intended to use 'as is' -

i.e. the client supplies all the necessary resources needed for the project and you are unable to add any further resources unless you unzip the package and create your own project- however in this scenario, you will no longer be able to create a return package- so worth checking with the client first 

I see that you have Studio 2014 so the following may look slightly different- (I no longer have this version installed) but please ensure the following is ticked

Does this resolve the issue?