The PDFCrowd PDF Writer is a separate extension, free to install, that allows you to generate PDFs using the (paid) PDFCrowd service (v2).
You need a PDF writer for generating invoices.
Sign up for PDFCrowd
Signup for a PDFCrowd account and subscribe to the v2 license. You can test with some free tokens, but will need a paid subscription when live.
Install the Extension
Using the modmore package provider, download the PDFCrowdWriter for Commerce package. It's available for free. Install it.
Once installed, go to Commerce > Configuration > Modules and find the PDFCrowd Writer in the list (tip: search for pdf).
Click on its name, and enter your PDFCrowd username and the API Key you can find in the PDFCrowd dashboard into the module options. Also enable the module in the test/live modes.