How to add “save to files” as an option

I have the Files app on my iPhone (and iPad) and I am struggling to save to it.

So if for instance I want to save a document or receipt and I tap on the square box with arrow at the top “save to files” does not come up as an option. How do I add this function in. I have spent ages reading articles on line but cannot find the answer.

I have the files app on the phone with folders created within. I do know this function has previously worked as I have items stored within those folders. But it no longer comes up as an option. How do I add it back in please.

I hope someone can assist.

Thank you.