I had the same issue and spoke with Shopify chat. They told me my Location was not saved. My address was entered, but the save button wasn't pushed. I went into Settings-Location and then hit Save and it worked right away. I was able to then purchase my shipping label after that. Hope this helps!
I am having this same issue out of the blue. I was able to create a shipping label for my last order. Now I am getting an error message. I have input all the information correct so i'm not sure why i'm unable to create a label. I am really stressed because this order is pressed for time. Could someone please try to help me resolve this issue?
Usually this is just Shopify timing out but you can try a few things.
1) try another browser (I know it sounds dumb)
2) try another browser and use PRIVATE BROWSING or INCOGNITO
3) Wait 24 hours and try again
4) Sometimes going in and click on SAVE under Locations kicks their system. ie: go to SETTINGS ---> LOCATIONS ---> click on your location and select the SAVE BUTTON. Sometimes their system gets disconnected from things. You normally see this with Facebook Sales Channel.
5) Contact Shopify Chat / Support right away. In some cases, your store specifically is having some type of error or issue with their system.