I thought the 2x4 were 10feet but they are only 8feet. I need 9feet.
is there a fastener that attaches two pieces of 2x4 end to end. Perhaps
a cleat?? I need to add a foot. So the 2x4 can run from post to post.

Probably best to go get a 10 foot 2x4 and cut it down. There are mending plates or tooth plates that can be used but they are not structural. Maybe someone else here knows of something you could use but it would probably cost more than a new 2x4.

I agree, you will have not structural strength splicing 2x4 together, cut down a 10' piece.

I agree with a full length, splice may cost as much as the board, If you have many, I would think you could take back as wood doesn't go bad? plus they will still make a sale.

Yes, return the 8' material. I personally would buy 18' 2x4's and cut them in half. (No Waste.)
Mike