Hello, how are you?
I'm a Google Cloud expert.
Here is my opinion about your trouble.
It seems you use Python3.x and Flask, Sqlalchemy to connect to database.
You have two instances one is Appengine(flexible environment) and the other is Google Cloud Sql instance.
Did you ever connect to db? I think no.
You must be create db engine from Google Cloud Sql instance name.
And create session from db engine.
In python3 use PyMySql.
I can not explain all here.
Just hope my bid will helpful for you.
Thanks.