Although you can still use pastry that hasn't puffed up properly, the eclairs won't look so good. If the dough is too wet (i.e. it doesn't hold its shape when you pipe it on to the parchment paper) this can happen. And that might be the result of using too much liquid or too little flour so make sure you weigh and measure everything out correctly. Another reason could be you opened the oven door during baking - it's really important not to do this! Another problem that can happen with easy chocolate pistachio eclairs is they can crack. This can happen if the oven temperature is too high or if you forget to sprinkle water on the parchment paper before adding the dough. The reason for the water is it creates steam in the oven during baking and this helps to prevent cracks. Having cracks isn't a huge deal because you can disguise them with the chocolate glaze afterwards anyway. These are important tips to know when learning how to make eclairs.