why I can not log on to icloud in my apple tv device but i can log on to my iphone or desk top?

That is an Apple TV (2nd generation). If you have two-factor authentication for your Apple ID account, then you may be asked to sign in with a special procedure: Type your password followed by the six-digit verification code directly into the password field. That can be confusing. Additionally, there is a time limit, after which the code becomes invalid.

Try this: First enter the characters of your password (do not submit yet), then manually request a new verification code, then append that code to the password (no spaces or other separating characters), and then submit.

What is your model Apple TV? Identify your Apple TV model - Apple Support