You cannot use this weapon. Bug

Bug Report ID: #913202244

Character name: LeapSlamAccident
Issue: Cannot equip 'Rapture Ruin Anvil Maul' despite all requirements being met.

Item requirements show green in inventory/stash.
Item turns red and provides "You can't use this item" error when moved to equipment slot.

Troubleshooting Done (Issue persists):
Verified attribute requirements are met.
Refunded/reallocated passive points.
Unequipped all gear and relogged.

Additional Information:
I was using this exact same item instance on this character without issue prior to the launch of the new league/patch.
The issue started immediately after the game updated to the current version.

They've set limit to 1 for some socketables, more than some actually. If that was your first loging after league launch that is the reason.
If you were already using it during new league time then i dont know.

Check 0.4 patch notes for item changes.