You only have 35 improvement slots, and you already have 43 improvements in that city. So you will not be able to buy any more improvements until you sell at least 9 improvements in this city, which would put you at 34/35 improvement slots used and allow you to build one.
Alternatively, you can buy more infrastructure to increase your number of available slots. Each 50 infrastructure that you have (rounded down) increases your available improvement slots by 1.