To Fix this, we recommend you re-import that product with the latest variant list. You can do this one of two ways:
Unsync the product + delete it from the destination store
Sync/ import it again from the source store to your destination store.
If you have already customised the product in your destination store. You will just need to recreate the variants and SKUs in the destination store product to match it with the source store.
Once this is done the product will be able to sync again.