Issue:

The input variable Generate code coverage files? is set to yes but I can't find the files.

Possible solutions:

The flags that are being used with the current input variable Generate code coverage files? are outdated. We have a ticket open to address this issue and update the Step.

In the meantime, you can use the following to generate these files:

Do not use generate_code_coverage_files. Make sure the value for this input is no.
Use xcodebuild_test_options to turn on coverage report generation by specifying: -enableCodeCoverage YES
Add a Script Step which can be used to view the coverage report:

    - script:
        inputs:
        - content: |-
            #!/bin/env bash
            set -ex
            # line coverage percentages for each target, source file, and function/method that has coverage information
            xcrun xccov view --report $BITRISE_XCRESULT_PATH            # get the list of files from result bundle
            xcrun xccov view --archive --file-list  $BITRISE_XCRESULT_PATH            # get line coverage for a given file
            xcrun xccov view --archive --file "$BITRISE_SOURCE_DIR/_tmp/ios-simple-objc/ios-simple-objc/ViewController.m" $BITRISE_XCRESULT_PATH

The script contains 3 different calls to xcrun xccov viewcalls:

The first call prints the coverage percentage for all targets.
The second call prints all the available files.
The third call prints coverage for a given file.