"Your postal code is incomplete" on Spark signup page
Just double-checking: when filling in the form, after typing the credit card number, do you see the postcode/zip field?

The payment processor (Stripe) displays a different field depending on the country (numeric ZIP for the US, alphanumeric for Canada, etc). I have to admit it's a bit confusing. I will double-check if we can do something about it.
If the issue persists, please do not hesitate to reach out by email at spark@civicrm.org.