Hey there AliV254,
It seems like you don’t have an Overwatch account linked.
If you want to post on the forums you must have an valid Overwatch license.
Make sure you’re using the correct account.
If you’re using the correct account, but you’re a console player.
Then please do check this thread: Unable to Post in Other Forums? Information here!
Also do make sure you don’t have any parental controls interfering.