There are many reasons why you are unable to use Splitit at checkout. Please review the article titled "Plan Creation or Card Update failure reasons" at this link https://support.splitit.com/hc/en-us/articles/21722191290514-Plan-Creation-or-Card-Update-failure-reasons to troubleshoot your case.
You can also review the articles about How Splitit Works at this link https://support.splitit.com/hc/en-us/sections/360005695500-How-Splitit-works for more information on creating a plan.