Hi Rob
I'm AnnaThomas and I'd be happy to help you with your question. In this Forum, we are Microsoft consumers just like yourself.
Sorry for the trouble this might have caused. There could be a few reasons why your PowerPoint design options have disappeared, but here are some common ones:
You might be using an older version of PowerPoint that doesn't have the latest design templates. In this case, you might need to update your version of PowerPoint to access the newest templates.
You are not a Microsoft 365 subscriber. However, please see the link below to troubleshoot designer in powerpoint
https://support.microsoft.com/help/83789fcb-f1ae-4703-b1b4-9c97259f592b
I hope this helps ;-), let me know if this is contrary to what you need, I would still be helpful to answer more of your questions.
Best Regards,
AnnaThomas
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