as @swarfendor437 says you have to purchase 18 Pro first.
If you already did that then make sure you remove all third party repositories before upgrading. See this thread: Upgrade Core 17 to 18 error messages unsure what to do
(BTW I upgraded 17.3 Pro to 18.1 Pro with no issues except that I was using an older kernel in 17.3 and needed to make sure to still select it in 18.1)