I can’t put the code sent to my email

For Apple ID passcode n am changing it but I can’t put the code sent to my email in verification boxes bc they keep disappearing what to do bc I can’t use phone until get the finder off right ?

I am trying to reset an old iPhone that I let my grandson use. I don’t know his password he doesn’t know the password it’s been a few years since he used it I for the Apple ID I get the code from the email but I can’t put into verification boxes because the verification page keeps disappearing and I can’t get it back, can anyone help me I’d appreciate it. I’ve been trying this all day and night.

[Re-Titled by Moderator]