Hi @Taner Mutlu,
You should be able to create your plan without a Bitbucket repository, at least until you can fix the issue with the Bitbucket<->Bamboo communication.
There are two types of communication happening between Bitbucket server and Bamboo:
Usually this error is related to some issue with the SSH communication. I suggest you checking this article where this problem is discussed in more detail: New access key is required but could not be added to Bitbucket Server.