I have built some software with a Mac OS target of 10.13. This was built on an M1 Monterry OS machine with specified arm64 and x86_64 architecture targets.
However, a user has reported they get a warning stating that it's not compatible with their 10.15 system and they need Big Sur (11).
Why would this be happening?
Projects and targets have separate deployment target settings. One of them probably has the deployment target set to macOS 11. Check the deployment target settings for both the Mac app target and the project and make sure they're both set to macOS 10.13.
This SO thread provides very clear explanations.
Key point is that
the version at "project" level serves as default, whereas the version at "target" level overrides the default value.
And the 2 may be different. Target may be lower of higher than project's
Here are examples from a test project :
In your case, you have define Project as 10.13, but Target is probably 11.0