Why are my contacts not syncing across devices

Try going to iCloud.com using a computer browser to see if the items are there and whether they are from the device or the computer. That should show whether the device or the computer isn't syncing. If the computer, try going to System Preferences/iCloud (Apple ID if you are using Catalina) and stop syncing. Wait a couple of minutes, then recheck syncing. If the device, go to Settings/Your Name/iCloud and do the above. As a last resort, you can also try signing out of iCloud and then sign in again.

IHave a iPad and an iPhone 8 how do I sync them together

You are welcome. Were the contacts all showing at iCloud.com/Contacts? Were you able to determine whether the phone wasn't syncing correctly or the Mac wasn't syncing correctly?

My problem is that I cannot get my iPad to sync with my iPhone I have a iPad air2 and a iPhone 8.

Did you try what I posted in my first post?