On your Ribbon, is the second tab from the left called Slide Master when this happens? If so, you're creating slide layouts, not slides. It's normal for slide layouts to display the slide number field as <#>.
Otherwise, please try running an Office repair:
- Click on the Start Button, then on Settings, then on Apps.
- Click on Microsoft Office 2019, then click on the Modify button.
- Choose Online Repair, then click on the Repair button.
- Grab a coffee, let the process complete, restart your computer and see if you can insert slide numbers normally.