I have a Google Pixel XL and the instructions are as follows:
Settings > Apps & Notifications > Advanced > App Permissions > Storage
There is also a short-cut.
in Apps & Notifications under 'recently opened apps' at the top of the page you can tap Pokemon Go then 'Permissions'.
Otherwise, their answer is accurate.
Pokemon Go needs permission to use your storage because it saves photos taken with the in-game camera to a 'Pokemon Go' folder in your phone's gallery.
Ergo those whose photos aren't saving should check their Pokemon Go permissions.
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