You can only increase your season rank during that specific season. Once the season ends, you can not longer increase that rank, and you can only claim rewards for ranks you unlocked.
Basically, because you didn't start playing until this season, you should not have been able to claim those rank rewards from last season, and that is also why you can't claim any other rewards from last season.
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Ok. Valid answer but then how come i was able to claim first 5 Rank rewards from last season. That also shouldn't have been possible. You could see that there is a tick mark till Rank 5.
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Unknown. Some sort of error in your favor is the best explanation I can come up with.
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Ok.. then i guess not possible to claim the Season 12 rewards