Whenever Gamefreak eliminates the possibility of some story-related Pokemon having their alternate coloration, Shiny Pokemon hunters have consistently expressed their hatred. You cannot make some ‘mons Shiny no matter how many times you reset them, unlike the Lake of Rage Gyarados in Gold and Silver.
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It has been a problem since previous generations, but more so during Generation V, when box Legendaries could never become Shiny and could only be obtained through giveaway events. The list of unsparkable Pokemon in Scarlet and Violet is no different.
Shiny Locked Pokemon list in Pokemon Scarlet and Violet
Since there is no shininess in the scripted encounters, finding a shiny Lechonk in the tutorial is impossible. Shiny locked Pokemon include: