What is the ruling on not praying in the mosque due to a medical condition in the feet, despite being able to walk, and is congregational prayer in the mosque obligatory or optional?

Congregational prayer is obligatory for men unless there is an excuse, and it is valid whether performed in the mosque or at home. If it is difficult for you to attend it in the mosque, then be keen to perform it in congregation at home. And whoever abandons it despite being able to do so has missed out on much good.