mail icon is not available

If the Mail icon is missing from the Dock go to the Applications folder and drag the Mail app into the Dock. That's all you have to do.

Try a restart, that fixes many small problems.

Open your Applications folder and look for the mail app. You can open it there or drag it to the dock and drop it there.

If there is no Mail.app in the Applications folder, you've manage d to delete a part of the OS and it will likely need to be reinstalled.

Click on Finder > click Applications in the sidebar > double click on the Mail App.