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While viewing the setting tab of your campaign and looking at the 'Ad Scheduling Options' do you see any notes there or question marks that you can hover your mouse over for more information.
I'm not sure if this is relevant to you or not but certain settings disable ad scheduling, like automatic bidding for instance.
Keep us posted if you can find out any more information.
Hope this helps!
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can you see this message below Ad scheduling settings: "Automatic bidding campaigns may not use ad scheduling"?
If yes you may consider switching to manual bidding if you feel OK to manage bids yourself. You can edit it in campaign settings - Bidding and budget - Bidding option - I'll manually set my bids for clicks.
Hope it helps.