@Borz It looks like you might have introduced an error into environment.yml while editing it.
I recommend totally removing fastai & reinstalling it to get the latest version:
cd ~
yes | jupyter kernelspec remove fastai
conda env remove -y -n fastai
rm -rf fastai
git clone --depth 1 https://github.com/fastai/fastai.git
cd fastai
conda env update
source activate fastai
python -m ipykernel install --user --name fastai --display-name "fastai"
source deactivate fastai
cd ~
When you update fastai in the future try running git pull && conda env update instead of editing environment.yml
fyi I’m updating Salamander’s AMI images today to use the newest fastai library: https://github.com/fastai/fastai_pytorch