Each section of your inventory or store has a limited amount of slots to put items in. For example a character can carry 9 primary weapons, 9 secondary weapons etc. and similarly for the armour items. Also the store area will have a limit. To clear space you will need to remove items by storing them or by disassembling / discarding them. Clear older / lower level kit by disassembling or use the store to pass it to another character.