most likely the flutter was not built by xcode cloud, try ci_post_clone.sh script
https://docs.flutter.dev/deployment/cd#custom-build-script
AndroDevs
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Same:
[!] Error installing PromisesObjC
[!] /usr/bin/git clone https://github.com/google/promises.git /Volumes/workspace/tmp/d20231016-10650-c4fygs --template= --single-branch --depth 1 --branch 2.3.1