My mistake, I wasn't the originator of that Feature Layer. I couldn't edit it in AGOL either. So, I added a copy and ran the script on it.
Now I'm getting a NameError.
NameError: name 'FeatureLayer' is not defined
But, here's how I have it:
feat_layer = FeatureLayer(feat_layer_url)
And the feat_layer_url is the new URL, which is valid.