I have numerous items to upgrade in my inventory, but I am unable to do so.
I have the materials (items, glimmer, shards).
Item to upgrade will be 265 or so.
Item I am consuming will be 270.
Item to be consumed will say "Base Power Too Low"
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What you're trying to use probably has a legendary mod on it, so if you take that out of the equation it's the exact power level of what you're trying to upgrade.