Participate more in the forum and it will work.
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yup, the same problem has been driving me crazy. hopefully it will work itself out for all os us =)
It's not a problem, it's a limitation that is built into the system, which helps deal with spammers. You just need to make some more posts, and the ability to make a signature line will become available in your profile. See here:
https://www.backyardchickens.com/t/622249/how-to-edit-signature/0_20#post_8641523