Why can’t I add legendary effects to this item?

Try the workbench’s Modify option, select the item, then look for its legendary mod slot. The regular weapon or armor mod list is a different menu. If the slot is there but still says zero, check that the modules are actually in your character’s inventory, not just stored away.

If you’re already in Modify and the modules are in your inventory, check the exact item you selected and whether it offers legendary modification. Then leave the workbench and reopen it, or switch servers and check again. A zero count by itself doesn’t tell you whether it’s a menu mix-up or a temporary display problem.

Also make sure the item is selected before judging the requirement. The available legendary options and costs can depend on what you’re modifying. If a normal supported item still shows zero with modules visibly in your inventory after reopening the menu, that sounds like a UI or inventory issue worth reporting.