You need to level up cause your trainer level is to low. Can only power up trainer level +2. Happy hunting and grinding.
Edit: and you need more stardust:)
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To be specific: Your Hariyama seems to be at level 27, which makes your level 25. However, the Hariyama is also of mediocre quality only, with IVs of 62% or lower. You might want to search for a better one before sinking stardust into a pokemon that eventually will become disappointing.
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Level up you filthy casual.. :)
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Downvoting this for fun.
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You try to power up but it says your level is too low to power up, and now you ask how you can power up? Could it be by reaching a higher level, so that your level is no longer too low? Just a thought...