If I understand correctly, you’re not able to create the bitbucket-pipelines.yml on Bitbucket Cloud. I think I found a screen similar to the one you described (having the messages “This repository does not have any files” and “You can still commit changes and run the pipeline but it may lead to a failure”), and the Commit File button worked just fine.
Maybe you can try another way to create the Yaml file.
Go your repository on Bitbucket Cloud, and click Source in the sidebar at the left.
Next, at the top of the page, click on the “…” menu next to the Clone button, and select Add file.
Enter bitbucket-pipelines.yml as the name of the file, and enter some dummy content, for example # foo, and click Commit file.
Go back to Source, click on the bitbucket-pipelines.yml file, and click Edit
Now you should be in some sort of “smart” editing mode, with a nice menu on the right, which lets you create pipeline configurations easily. From the Change the template menu you can select a template pipeline, and then in Add Pipes menu you can discover the snippets to use with SonarCloud pipes.
I hope this helps, let me know at which step you get stuck!