Streamlit Deploy: “Unexpected error” when setting App URL (even if optional field is empty)

I am trying to deploy my Streamlit app using streamlit.io cloud.
On the “Deploy an app” page, when I enter my GitHub repo, branch and main file it shows an Unexpected error under the “App URL” field.

Even if I leave the App URL empty (it says optional), I still get the same error and the Deploy button is disabled.

I tried changing the repo name, using different URLs and different branches but the error still happens.

Here is a screenshot of the error:

As you can see in this pic

Why does Streamlit Cloud show “Unexpected error” in the App URL field and how can I fix this?