I'm trying to log into my Apple ID but I forgot the code, what to do?

I do not know of any "iCloud" code. Do you mean a password?

This Apple article has instructions for what to do if you have forgotten your Apple account password —> If you forgot your Apple ID password - Apple Support

or go directly to this link —> http://iforgot.apple.com/

If you don't receive your verification or reset email, read this Apple Support article: If you didn't receive your verification or reset email - Apple Support

"When you make certain updates to your Apple ID, we’ll send you an email to help you complete the update. For example, you might have reset your password, reset your security questions, or added a new email address to your account."

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community

If you still need help this page provides country-specific Apple Support contact information for customers seeking help ➞ Contact Apple for support and service - Apple Support

This page has telephone numbers for contacting Apple for support and service ➞ Contact Apple for support and service - Apple Support

If you are signed into this forum using the same Apple Account then your browser may be configured to automatically enter your password. You should be able to view stored passwords in your browser's preferences/settings.

Again, in 20 years of having been on this forum and using iCloud I have never heard of an "iCloud security code".

Assuming what you mean is a two-factor authentication verification code, from: Get a verification code and sign in with two-factor authentication - Apple Support

A code is sent first to your trusted devices automatically:

"If you have a trusted device running iOS 9 and later, OS X El Capitan and later, iPadOS 13 and later, or watchOS 6 and later, the verification code is displayed automatically on your trusted devices."

If you don’t have a trusted device handy, you can choose to have a verification code sent to your trusted phone number as a text message or phone call.

1. Click "Didn't get a verification code" on the sign in screen.

2. Choose to have the code sent to your trusted phone number.

3. You'll get a text message or phone call from Apple with your verification code. This text message might include an additional domain validation line. This line includes the @ symbol, the website name, and your code (for example: @icloud.com #123456 %apple.com).

4. Enter the code on your other device to complete sign in.

"If you can’t receive a verification code on your trusted devices automatically, you can get one from Settings, even if your device is offline."

On a mobile device go to Settings > [your name] > Tap Password & Security > Get Verification Code.

On a Mac: Go to Apple menu  > System Preferences [System Settings], then click Apple ID. Click Password & Security > Get Verification Code.

"If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account [ How to use account recovery when you can’t reset your Apple ID password - Apple Support ]. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity."

By the way, it is a good idea to have more than one way to get a code.