Hello Alpesh Panchal,
You can find some helpful information, as well as the requirements to change the email address for your Apple ID account here: Change your Apple ID
It seems like you may be trying to use an email address that's associated with another Apple ID or iCloud account, is that correct? If so, please check out the details in the "If you can't change your Apple ID" part of the above link:
If you can't change your Apple ID
If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.
If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it.
In some countries or regions, you can use your phone number as your Apple ID. But if your Apple ID is an email address, you can't change it to a phone number.
Thanks!