unity 2020 animation loop once code example

Example: how to loop an animation in unity

Its probably too late to help you with this, but just incase anyone else has this issue, your looping options are greyed out because your animation from the asset store is read-only, select your animation in the project window, and press Ctrl+D to duplicate it, and you should be able to now set the looping options on the new animation as the other answerers have described.