Trying to create a variant: "Error - SKU can't be blank"

[sadsfg]

I'm trying to create variants for a product and every time I try to save, it says "There is 1 error with this product: SKU can't be blank" then deletes my variant. This has never been an issue before so I don't understand why this is a problem now all of a sudden.

I usually create the variant first from shopify, leave it blank, then go to a third party and connect a product from there to the new variant, so the SKU creates itself at that point. I can't remember being mandatory to enter a SKU. Help?

[jam_chan]

The same. It happened before. But it only happens randomly. Most stores can work well. Only a few store gives this error. I just wonder what causes this error.