What lvl of warehouse storage do I need to upgrade my castle lvl 20 to lvl 21?

To level up from castle 20 to 21 your storage needs to be:

Food Storage: unit 1 to level 10, unit 2 to level 9

Ale Storage: unit 1 to level 10, unit 2 to level 4

Wood Storage: unit 1 to level 10, unit 2 to level 10, unit 3 to level 1 (to accommodate the amount of wood needed to upgrade the iron storage units so upgrade these first)

Charcoal Storage: unit 1 to level 10, unit 2 to level 4

Marble Storage: unit 1 to level 10, unit 2 to level 9

Iron Storage: unit 1 to level 10, unit 2 to level 10, unit 3 to level 1

Please check the Castle Upgrade Requirements guide to see how much of each resource you need and then check your warehouses.