If your Shopify store uses multiple locations you need to ensure your default shipping location is set to fulfil orders otherwise you will encounter issues when trying to add a custom sign to your cart.
1. Login to the Shopify Admin
2. Click on "Settings" in the bottom left corner. A popup will appear.
3. Click on "Locations" in the left sidebar. Make note of the name of the location under "Default location" on this page. We'll need this in the next steps. My default location name is "Test location 2" in this example
4. Click on "Shipping and delivery" in the left sidebar. Then click on "General shipping rates"
On this next page you will see one of two possible screens. For this step follow the one that you see.
5a. If you see a "Fulfilment location" header, click on "Add locations", then select your Default location (from step 3) and press "Add"
5b. If you do not see a "Fulfilment location" header, you will see a "Not shipping from these locations" header. Find your default location and select "Add rates". This will prompt you to set your shipping rates. Press "Done" when finished
6. Save this form
That should be it! After you've saved these changes you should be able to add products to your cart