How to solve TypeError: arr[Symbol.iterator] is not a function in my React project

The way you're consuming the return values in the IndividualProject component of your useSelectedProjectValue custom hook is wrong:

const [projects, setProjects] = useProjectsValue();

should be

const { projects, setProjects } = useProjectsValue();

React uses array destructuring e. g. for their useState, but your useSelectedProjectValue hook is rather returning an object and not an array, so you have to consume it in the components with object desctructuring. Or change the return value of the custom hook to an array.

If you want to learn more about this topic I recommend reading Kent C. Dodds' blog: https://kentcdodds.com/blog/react-hooks-array-destructuring-fundamentals