Why can't I download my shipping label?

Shipping labels are only created for sellers that are using Poshmark's own PoshPost shipping labels. 
https://poshmark-business-seller-knowledge-base.groovehq.com/help/what-are-my-shipping-options-on-poshmark

If you have chosen to use your own shipping labels, you will not be able to download the shipping label from any provided links, nor receive the shipping label by mail. If you are expecting to use PoshPost and still are not able to access your shipping label, please reach out to your Enterprise Support team.