Why Can't I Donate a Tech Level 5+ Item?
As you open a slot for an ally, you also open a slot for a referral to provide bonuses. Referral slots are capped at 50 item sets since it is harder for players to acquire referrals than allies.
For example, if you have 350 INFRASTRUCTURE, you create openings for 70 allies and will mean 70 item sets are set aside and the lesser of 70 or 50 will also be set aside for your referrals. This becomes 70 and 50, making 120 item sets.
Only items of TECH LEVEL 5 or higher are set aside. Note also that one of each of the highest TECH LEVEL items are set aside into STORAGE for you to use when your level increases sufficiently.
There is a quick reference to this within the REFERRALS [MORE INFO] tutorial on the INVENTORY BONUS (fifth page) and please see the related help topic on Referrals here: http://www.zombiemoongame.com/HelpDesk/knowledgebase.php?article=7