Unable to upgrade to cPanel & WHM version 88 when running the "/scripts/upcp" command, or via the WHM interface Home >> Home >> cPanel >> Upgrade to Latest Version is because cPanel uses a tier system that allows administrators to configure how servers are upgraded. The following tiers are available:
As of this writing, cPanel version 88 is on the "Current" and "Edge" tiers. Up-to-date information can be found here: http://updates.cpanel.net/.
To upgrade to cPanel & WHM version 88, go to WHM >> Home >> Server Configuration >> Update Preferences and select the appropriate tier, which has the cPanel desired version and run the upgrade process.
Detail information about the release process and tier information can be found in the following link: https://docs.cpanel.net/knowledge-base/cpanel-product/product-versions-and-the-release-process/.