What type of data is expected in your database field?
In any case, please use prepared statements to avoid type errors like :
INSERT INTO [Test2] (rate) VALUES (?)

Ejs:

<form method="POST" action="/">
    <label class="container" >5
        <input type="radio" name="_rate" id="_rate" value="5" required >
        <span class="checkmark"></span>
   </label>
   <label class="container">3
       <input type="radio" name="_rate" id="_rate" value="3">
       <span class="checkmark"></span>
   </label>
   <label class="container">1
       <input type="radio" name="_rate" id="_rate" value="1">
       <span class="checkmark"></span>
   </label>
</form>

Nodejs:

const express = require('express');
const mysql = require('mysql2');
const app = express();

const db = mysql.createConnection({
    host: //'localhost' or you hostname,
    user: //'root' or your username,
    password: //yourpassword,
    database: //'database name',
})

app.post("/", async (req, res)=>{
    const _rate = req.body._rate
    await db.query("INSERT INTO Test2 (rate) VALUES (`${_rate}`)", (err, result) {
    if (err) {
       //Handle error
    } else {
       //Do whatever you want with `result`
    }
})

app.listen(3000, ()=>{
   console.log('Server started working on port 3000')
})