Probably is because the fast.ai repository don’t have the dataset already, you have to download. You can find the official dogscats dataset here.
One of the easiest ways of to do this is open a code cell in the notebook and run this commands:
!wget http://files.fast.ai/data/dogscats.zip
!unzip dogscats.zip
!mkdir data
!mv dogscats data
Run the cell with this lines of code in your notebook and everything will run.
Ps: Don’t forget to already have installed unzip library.