How do i select items in a gallery usign checkboxes

i'm having problems with a gallery in powerapps, i'm trying to select multiple items in the gallery using checkboxes, now i can select just one item from the gallery, i've tried a couple of things an nothing seems to work. I need to select more tahn one so i can print multiple items in PDF.

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I tried with Updates, ForAll, Patch, both in on check and on select but nothing seems to work, everything keeps giving errors.