Here are the probable reasons explaining why you may not be able to shop more with Tabby:
If you've made your first purchase with Tabby but haven't yet paid for it, you won't be able to make additional purchases until you've made payment for the first one. Once you've paid for your first purchase, your credit limit with Tabby will be unlocked, and you'll be able to make more purchases using this limit.
If you already have a credit limit with Tabby, but it's not enough to cover your new purchase, you may want to consider making a smaller purchase.
If you have overdue payments with Tabby, you won't be able to spend more until you've paid off the overdue amounts.