Type Error: res.status is not a function supertest

`TypeError: res.status is not a function

49 | })
50 | .catch(error => {
> 51 | return res.status(400).json({
| ^
52 | success: false,
53 | error,
54 | results: null

at status (Routes/Controllers/auth/index.js:51:18)`

`router.post('/signup', (req, res) => {`
`let validate = ajv.compile(signupSchema);`
`let valid = validate(req.body);`
` if (!valid) {`
`return res.status(400).json({`
`success: false,`
`error: sumErrors(validate.errors),`
`results: null`
`});`
`}`
`auth`
` .signup(req.body)`
`.then(results => {`
`console.log(res.json);`
`return res.status(200).json({`
`success: true,`
`error: null,`
`results`
`});`
`})`
`.catch(error => {`
`return res.status(400).json({`
`success: false,`
`error,`
`results: null`
`});`
`});`
`});`

Link to the project [online-judge](https://github.com/NBNARADHYA/online-judge)

When testing, with supertest this problem seems to exist.

But the same code works perfectly fine when I start the server manually using npm start and when requests are sent using postman.