Amazon FBA program guides sellers to align their inventory supply with buyer demand, and they check if there is sufficient inventory capacity for the given items.  When you see this message, it means that you have (or Amazon has) reached the maximum inventory quota allowed for this product, due to capacity or other restrictions determined by Amazon's automatic logistics algorithms.  If you feel like this is incorrect, please reach out to Amazon. Sometimes they can lift these restrictions for sellers who reached out depending on the situation.