I am trying to practise packaging design. Can someone pls help me with this

have a look at this post mate

https://community.adobe.com/t5/dimension/fix-for-decal-being-cropped-when-wrapping-around-object/m-p/11831732#M4980

that image is a decal so the first thing you need is to use a fill instead... then I would select just the outside edge of the can with the magic wand so you can export the Uv map needed

p.s, your title says you want to learn package design [which this isn't] so I'd also goto https://www.fantasticfold.com/ and grab the beta

Sorry for the typos