Hi Trish,
We understand you can’t see the Design Ideas in the side pane when you create a presentation.
First, PowerPoint Designer is a subscriber-only, Microsoft 365 feature. You should be a Microsoft 365 subscriber to use this feature. If you have Microsoft 365 subscription, may I know whether you can see Design Ideas button under the
Design tab?

If not there, we suggest you go to PowerPoint-> Options-> Customize Ribbon, make sure you check the Design tab option. (Please confirm whether Design Ideas is under the Commands tab.) If you can’t see any contents in the Design Ideas,
it may be due to some requirements of PowerPoint Designer for Windows. Please refer to this article:
PowerPoint Designer

Moreover, could you provide the following information for us to move further?
1.
The full screenshot of the PowerPoint-> File->Account->Product Information page.

2.
Can you see this feature before?
3.
Can you check whether it can work in the new presentation?
Best Regards,
Ivy