Hello HomerJ,
There is no direct upgrade route from ALM11.5x and prior versions to ALM 12.6x versions. In order to upgrade an ALM 11,5x project within ALM12.6x you must first upgrade it to an ALM 12.xx version first and then it can be upgraded to ALM 12.6x.
Directly from the installation and upgrade guide for ALM12.6x:
From version: To ALM 12.60:
ALM 12.00 -12.5x Upgrade projects directly to ALM 12.60.
ALM 11.52 Upgrade to ALM 12.53 or 12.55, then upgrade to ALM 12.60.
ALM 11.00 Projects must first be migrated to ALM 12.21. For details, see the ALM 12.21 Installation and Upgrade Guide.
You will need to upgrade that project to one of the above listed ALM12.xx versions first in order to make it compatible to upgrade to ALM12.6x version.
Hope this helps,
Dan