Hi,
It's maybe linked to the error you got ? If you need to pay first to get the label with the interface provided by MyParcel, you also probably need to pay first to be able to use the "print label" button in the HikaShop interface.
Now, it's been more than a year since last time I used the interface on a customer's website, so I'm not very familiar with all the inner workings of MyParcel, especially since everything is in Dutch and I don't speak it.
What I can say is that the plugin works fine for the many merchants already using the plugin for what they are doing. If the "print label" button wouldn't work, many others would have told us before.
Maybe you need to check with MyParcel support ? They can probably shed some light on why it's doing that.