Why I cannot set meals in reservation detail?

It is very likely that you have not defined the price of offered meals for the booking dates. First open the table with permitted meals and prices in the Pricelist / Meals module and check the given dates.

Enable the offered meal options and enter prices for individual guest categories for the specific dates directly in the table or by visiting Group settings or Add a new season. After saving the settings and returning to the booking, the meal option should appear in the booking.