By default no singer will be selected for new tracks.
Select a singer from the dropdown in the arrangement panel.
You can also select the singer from the Voice panel. This also allows you to choose a specific version of the voice database, if multiple are installed.
Pro Feature - Cross-lingual Synthesis
If an AI singer is selected, you may also need to select the appropriate language for your song. AI voice databases have a default ("native") language, but can leverage cross-lingual synthesis to sing in other languages as well. Japanese, English, Mandarin Chinese, and Cantonese Chinese are currently supported.