What will happen if I do not use canonical tag ???

if i don't use canonical meta tag for website.... search engine don't rank my post?

snowmanstudio said:

if i don't use canonical meta tag for website.... search engine don't rank my post?

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You will rank, but your rankings will be fluctuating, plus that's a guarantee you will never rank #1 if it's a keyword that's worthwhile.
The biggest social network in my country doesn't have a rel canonical tag for it's pages.

Google will sandbox you for 100 years

Canonical tag is no mandatory and there are plenty of websites which rank without them.

You only really need it if you have the same page which has 2 urls for example one post goes into 2+ categories, like site.com/category1/page and site.com/category2/page. Which page is google supposed to rank? Canonical tells search engines which copy to rank.
Otherwise, as mentioned above... google will not know so it will switch from page 1 and page 2 thus unstable rankings.

Google will consider your content duplicate.

You run the risk of many of your pages being viewed as duplicates when they're not.

This can cause havoc with your rankings, in fact in many cases depending on how competitive your niche is your pages might not even rank at all.