use state to build url query string - next.js

Question

I am new to Redux, so any help would be appreciated.
I want to add a variable to my fetch GET request URL inside the action creator.
yourapi.com/getuser/{user1}
I might not be following the correct process, I am very new to redux. I am using NextJS with React-Redux for this project.
My action:
// Get User Object
export const load_user = () => async dispatch => {
try {
const res = await fetch(`/api/getuser`, {
method: 'GET',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json',
}
});
const data = await res.json();
if (res.status === 200) {
dispatch({
type: LOAD_USER_SUCCESS,
payload: data
});
} else {
dispatch({
type: LOAD_USER_FAIL
});
}
} catch(err) {
dispatch({
type: LOAD_USER_FAIL
});
}
};
That part seems ok.
In this getuser.js file, the action calls (The action creator) how do I append a username variable onto the URL ${API_URL}/GetUser/{username} ?
export default async (req, res) => {
if (req.method === 'GET') {
const username = ??????????
try {
// How to get username???
const apiRes = await fetch(`${API_URL}/GetUser/username`, {
method: 'GET',
headers: {
'Accept': 'application/json',
}
});
const data = await apiRes.json();
if (apiRes.status === 200) {
return res.status(200).json({
user: data
});
} else {
return res.status(apiRes.status).json({
error: data.error
});
}
} catch(err) {
return res.status(500).json({
error: 'Something went wrong when retrieving user'
});
}
} else {
// Error. Not a GET request. They tried POST or PUT etc.
res.setHeader('Allow', ['GET']);
return res.status(405).json({
error: `Method ${req.method} not allowed`
});
}
};
I tried
const user = useSelector(state => state.user)
but I get the error
Invalid hook call error - TypeError: Cannot read properties of null (reading 'useContext')