You are using a Resnext Model and dont have the required weights. Check here for more info: Link
There are no set location where datasets need to be stored. Jeremy ideally stores the dataset under his home directory and symlinks where ever he needs access to.
You should download the required files from kaggle (Link) and put them under the data directory with relavant datset name in the location as fastai library or provide the appropriate path when loading the data in code or symlink the dataset location appropriately.
i.e data/planet/[Kaggle files here]
P.S : Do Search the forums before starting a new thread as most of the times you can save time and find the answers to your questions easily.