error: pathspec 'v5.15.137' did not match any file(s) known to git.

I followed the following steps to build kernel code. But it reports that it cannot find v5.15.137 branch.

Can you help check how to solve this?
 
git clone https://github.com/intel/linux-kernel-overlay.git
cd linux-kernel-overlay
git checkout lts2021-ubuntu
./build.sh

= [up to date] mainline-tracking-overlay-v6.7-rc3-ubuntu-231129T100251Z -> mainline-tracking-overlay-v6.7-rc3-ubuntu-231129T100251Z
= [up to date] mainline-tracking-overlay-v6.7-rc3-ubuntu-231206T020418Z -> mainline-tracking-overlay-v6.7-rc3-ubuntu-231206T020418Z
= [up to date] mainline-tracking-overlay-v6.7-ubuntu-240110T080436Z -> mainline-tracking-overlay-v6.7-ubuntu-240110T080436Z
+ git checkout v5.15.137
error: pathspec 'v5.15.137' did not match any file(s) known to git.
+ popd
~/test3/linux-kernel-overlay
+ pushd /home/ubuntu/test3/linux-kernel-overlay/build/linux-kernel-v5.15.137
~/test3/linux-kernel-overlay/build/linux-kernel-v5.15.137 ~/test3/linux-kernel-overlay
+ rm '/home/ubuntu/test3/linux-kernel-overlay/build/linux-kernel-v5.15.137/../*.deb'

Start with build.sh on the lts2021-ubuntu branch and trace the git checkout v5.15.137 step that fails after cloning and fetching. Check which refs are available and compare them with the expected build/linux-kernel-v5.15.137 path; done means the documented build steps complete without the pathspec error.

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