Inherent Synchro, Xyz, and Link Summons can be cancelled. However, Inherent Summons on Main Deck Monsters, (e.g. Chaos Dragon Levianeer) and Inherent Summons on Extra Deck Monsters that aren’t the conventional summoning method cannot be cancelled.
For example, Ultimaya Tzol’kin’s summon cannot be canceled because it is not a Synchro Summon.
“Cannot be Synchro Summoned. Must be Special Summoned (from your Extra Deck) by sending 2 Level 5 or higher monsters you control with the same Level to the GY (1 Tuner and 1 non-Tuner).”
This creates weird situations where it makes it harder for duelists to navigate the game, since they have to think about which summons can be cancelled and which cannot, and when playing under a timer duelists are clicking fast, a lot of misclicks may happen and being able to cancel a summon before choosing the materials is very valuable, especially for consistency purposes.