Very odd! My Chrome is up to date and I had no luck trying this on other computers (albeit on the same network, so can’t discount that the network might be blocking it somehow). I get the same page error when I try Microsoft Edge. I also tried using my phone and going on the cellular network and turning WiFi off, same error. It might be helpful to know that I was able to install the app without issue last year before deleting it as it didn’t serve my needs then.
The reason I am wanting to use the app is that the Edit Packing Slip Template functionality does not seem to work for printing out the Cart Attributes such as a Card Message, Delivery Date, etc. which I have implemented code for (and have no issues seeing these custom fields filled in by customers in respective orders). It worked well to print the Order Note when I used the code for the standard Order Notes that are enabled through the Cart Page but I abandoned that as many customers did not click the text to get the text field to appear. In my searches it appeared that the Edit Packing Slip Template is restrictive in what it can print out even if I implement the correct code… leading me to try my luck with Order Printer (but even then, I forget if the template code was customizable in Order Printer - may have been why I abandoned it last year!).