I had the same problem with one of my apps this week. Try to do so (I give you an advice I hope it works ):
1)Try to change browsers when you upload the update of your app (Firefox,Chrome,Opera,Safari…)
2) Try to create a new certificate and a new Provisioning profile and after upload your app.
3)The staff of Thunkable is always doing a great job, and it could be that sometimes the server could have some little problems due to some updates they are doing or maybe it is busy for the many applications that are sent, wait and try to send your app after some days.
4)Check into your app that there are no problems, check the Thunkable guidelines