Why am I seeing a 'fulfillment failed' error in my online store?

I was wondering if anyone has some advice on a recent problem we have encountered.
we have had our store for over a year and never have seen this error before. When we try end mark our orders as Fulfilled we get a little red error box after several seconds that says “fulfillment failed”.
I appreciate any advice you have on this.

We are also experiencing this issue. We also can’t save any changes on product pages. Not sure if customer page changes are affected as well.

I am having this issue right now as well! I cannot fulfill orders and my customers cannot add products to their carts!

I also just realized that items cannot be added to the cart. Sounds like a Shopify issue more than something we did wrong