PowerPoint's automatic accessibility checker does offer some support, but it does not find all accessibility errors. Our support articles will help you prepare your presentation so that it can be exported to an accessible PDF document in accordance with the PDF/UA Standard. For an initial overview, read the article Quick Start Guide: Preparing a PowerPoint Presentation for PDF Export with axesSlide.
Nevertheless, it can help to start with the automatic check. To do this, select the Review tab at the top of the ribbon and click on the Check Accessibility button.
The Accessibility pane then opens. Errors and warnings are listed there. You should any rectify errors. In the case of warnings, you need to decide if the check has found a genuine error or if it is a false positive. If you click on an error or warning, you will be shown further information and steps to rectify the error.