Cube won’t upgrade rare items

Perhaps upgrading a rare to a legendary is kind of a useless thing to do but it’s listed as a quest requirement at the start screen. Anyway, the cube won’t upgrade any rare items. All of the the required mat items are in the cube as per spec.

Would be so kind to post a screenshoot (imgur et. al.) of what exactly you want to upgrade ?
The wording “mat items” puzzles me quite a bit.

The rare item you wish to upgrade has to be level 70.