Power up

Don't power up Ninetales, it's useless.

Either way, at L24 you can only power up to a L25.5 Ninetales (the cap is 1.5 levels above your trainer level, for all Pokemon).

As was already mentioned, you can only power up your Pokemon up to 2 levels above your trainer level. So once you level up, you could power it up twice more (each power-up is half a level).

That said, as cool as I think Ninetales looks (it's one of my top favorites of any generation), sadly it doesn't perform very well in battles. So unless you have a TON of Stardust, I wouldn't invest into Ninetales as it gets outperformed by a lot of other Fire type mons.

Just do a Ninetales raid when sunny and you can get a level 25 one.

You probably caught the Vulpix at level 26. Once you reach level 25, you can power it up twice (to level 26.5 and 27).

Flareon, Arcanine and even Charizard are far better battlers than Ninetales, not to mention the legendaries (Moltres and Entei).

As much as I love the design of Ninetales, it's one of my favorites from gen 1, it is not a pokemon you should power up. Flareon is easier to get and so much better as a fire pokemon in POGO that it's not even funny.

Since you're level 24, it's unlikely you have much stardust anyway, so you should be careful with using it. By leveling up your trainer you can still find stronger and stronger pokemon in the wild up to trainer level 30, so you should really not power up anything but instead stick to upgrading your team by catching better wild mons and evolving them.