getting a pokemon from database but i cant level it up

[AiriSan]

I'm trying to get a pokemon using phex and I read using the database worked best, so I tried with metagross but the only ones I find that work are beldums, but when i try to change the pokemon from beldum to metagross it pops up the not valid mark.

the reason only beldums work is cause the metagrosses wont let me change their IV,s, Personalities and their Ability.

I don't know if I wrote that well enough to understand but I hope I did

[Desmondcsz]

1. What level is the Beldum? Metagross needs at least level 45 so if Beldum is at a lower level you can't just change it to Metagross like that.

2. Depending on the game, or the origin of the mon, you can't just freely change IVs, Nature and/or Abilities.

[Atrius97]

If you are unable to change the IVs, Nature, etc. it's likely due to PID Mismatch, the RNG restrictions of the game do not allow you to change stats freely, they must all match each other.
See this page for tutorials on how to solve this issue:

You may also want to install PKHeX-Plugins and simply use AutoLegality to solve it for you.
https://github.com/architdate/PKHeX-Plugins/wiki/Installing-PKHeX-Plugins