You can perform Friend Point Summon even if your Servant, Craft Essence, or Command Code inventory is full.
However, there is an overflow limit of 100 slots. When either your Servant, Craft Essence, or Command Code inventory reaches the limit (full, plus 100 items), Friend Point Summon will no longer be possible.
You can perform Saint Quartz Summon whether you have reached the inventory limit or not.