Foreign collection doesn't seem to be appearing in aggregated data
You are writing 'User' as collection name, please check saved collection name in MongoDB. It should be saved as 'users' collection. So, you have to write 'users' in lookup like below code:
router.get('/forum', async (req,res)=>res.render('forum', { newPost: await Createpost.aggregate([ { $lookup: { from: "users", localField: "postUsername", foreignField: "username", as: "postUser" } }, { $sort: {date: -1} } ])}));