I have products with the same variation. From product edit form I can only add new variation. In variations field form setting i haven't "Allow users to add existing products."
how can i have this option or How can add existing variations?
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The option you're looking for is not present on purpose (and the widget itself is different despite how it worked in Drupal 7). Every product has its own variations that cannot be shared, so there's no way to reference the same variation from multiple products.
If you need to sell the same thing from multiple contexts, what you probably need is some other entity that references the same product, not a standalone variation. Products:variations have a very clear parent:child relationship ... they're "variations" of the main "product".
I'm not as familiar with D8 commerce but should be the same. If you edit the product field on your product display content type you can tick "Allow users to add existing products."
That will allow the option to add existing variants