Hi there JessWhiz,
If you are having issues pasting text into Keynote, first ensure you are following the relevant section here to complete this task: Copy and paste text in Keynote on Mac
If you are following those steps and this continues, then let's next ensure your Mac is fully updated as well. This installs feature improvements and bug fixes that can resolve unexpected behavior like this. These steps from Update macOS on Mac show how to back up and update:
Before installing new software, it's a good idea to back up your Mac.
1. From the Apple menu in the corner of your screen:
• Choose System Settings. Then click General in the sidebar, and Software Update on the right.
• Or choose System Preferences. Then click Software Update.
2. If Software Update finds new software, click the Update Now or Upgrade Now button to install it.
3. Enter your administrator password, if asked. During installation, your Mac might restart, and you might see a progress bar or blank screen several times.
If this continues after updating, please provide more details on what exactly occurs when you try to paste text from Pages and what kind of text is being copied. We'll use that info to assist further.
Cheers!