Hello jboyiddle,
Thank you for using Apple Support Communities.
I understand that you are trying to restore an iCloud backup from your iPhone 8 Plus to your iPhone XS Max. But you are not able to because the backup is from a higher iOS version and you can't update the XS Max at this time. Until a higher version of iOS is available for the XS Max, you will not be able to restore the iOS 12.3.2 backup.
What you can do to get around this is to sync your device to iTunes or iCloud to move any saved data from your computer or iCloud to your device. To sync items with iCloud, sign in to iCloud on your device in Settings > Your Name > iCloud. You can find the steps to sync with iTunes in the following article:
Sync your iPhone, iPad, or iPod using iTunes on your computer
Additionally, if you were using iCloud Photos and Messages in iCloud, this data will automatically sync to your device when you turn them back on in Settings > Your Name > iCloud on your device.
Keep all your messages in iCloud
Access and view iCloud Photos on your iPhone, iPad, or iPod touch
Best regards.