How do I further power up my Pokémon

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:)

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.

Level up you filthy casual.. :)

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...