Based on the pictures, the self.data attribute will be created inside the generate method, so you would need to call it first:
dataset = MolecularDataset()
dataset.generate(args)
print(dataset.data)

Also, note that line34 has a condition to create self.data, which is then used in line37+ without a guard, so it could easily fail.
PS: you can post code snippets by wrapping them into three backticks ```, which would make debugging easier and would allow the search to index your post.