May be you have created the UiPath apps project with vb. In the Vb there is no option Value binding.
Follow the below steps to store the dropdown selected value in a variable -
→ Select that dropdown and in the right go to events → Edit rule → Click on + icon → Search for set value → In Item to set field create a variable to store the selected option in the dropdown → In the Value field click on Expression editor → Then give MainPage.Dropdown.SelectedItem.toString.