I told the waiter/waitress I wanted to take home the rest of the food I couldn't finish at the restaurant, but he/she declined to let me.

It is the restaurant's decision whether or not to allow customers to take home food and other items served at the restaurant. In recent years, more and more restaurants have started to allow take-out from the perspective of reducing food loss, while some restaurants do not allow take-out due to food sanitation issues. If you are at a restaurant that does not allow take-out and you really want to take home leftover food, you should discuss the matter with the restaurant manager again, telling him or her that you will eat it promptly after you return home or reheat it before eating it, and that you will do so at your own risk.