I don't see anything in the User Manual that explains the situation you are having.
It is possible that you have inadvertently set something that is causing the situation.
I suggest you do a camera reset.
See page 35 in the User Manual.
If that does not fix the situation, it is time to call Olympus Service.
They may have encountered something like this before.
..
I'm having issues with an Olympus SP570UZ digital camera. I'm wanting to set my ISO at 200, but the camera won't let me.
I do all my shooting in M mode, and have ISO set to manual. The ISO selections are thus:
Auto, H.Auto, 64, 100, 200, 400, 800, 1600
But 400, 800, and 1600 are the only settings the camera will let me choose. Auto, H.Auto, 64, 100, and 200 are all grayed-out and I cannot select them. Auto is only available in S, A, and P shooting modes, with H.Auto, 64, 100, and 200 still grayed out in those as well.
I really need these lower ISO settings because at 400, my pictures are fairly noisy.

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