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

[secondspecs]

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.  

[MEDHealthChoice]

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.

[prettyandall]

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

[secondspecs]

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