Is it okay to use "Please" in validation messages?

Don't overuse it. "Please fix errors in your form" is polite, "Please" on each field validation error is just unnecessary and annoying.

Passable, but avoid.
You can be polite without resorting to pleading.

Similar to adding the suffix “ because you suck.” at the end of the message. It’s better if you don’t.

No you have to use どうぞ in order to transition this shitty cyberpunk future to a slightly less shitty cyberpunk future

Also I love it when user report a problem with an validation error like "Please enter an item id" and the user has no clue what to do to fix the error. Happens more often that it should -.-

Plain simple text:
password is incorrect
email is wrong
ect.

@Gregozor2121 "Your password doesn't match leenucks4lyfe69"

"Make sure your password is correct, or else ..."

___ must be between {min length} and {max length} characters long
___ is required
___ can only contain {data type}*

*If your ___ contains other values, please contact support <— in case you didn’t read up on global conventions for a type of data and naively assumed it was consistently in the form you are accustomed.