I am at level 15 and I want to upgrade my castle to level 16, but I'm stuck with GOLD storage on 1..

There is no cap on gold storage, it will be increasing. The numbers shown at the top of the screen will show 1.0M until your amount raises to 1.1M which make take some time depending on the production rate.

If you have gold stored in reward chests in the items list then you can use this at any time to add to the amount shown at the top of the screen. You can tap on the amount at the top o the screen to see your total and at the bottom you can see your 'Use Item (Stock...) value that will tell you how much extra you have stored away.