"TypeError: Converting circular structure to JSON" with GET request

You can just use promise fulfilment to get the results.

server.get('/strutture', async (req, res) => {
    //GET STRUTTURE
    try{
        const strutture = await db("structure");
        res.json(strutture);
    } catch(err){
        console.log(err);
    }
})

This will return the correct query results