The "TuneCore Direct Advance" button will appear if you have been pre-selected for an advance by TuneCore. You can find the criteria for eligibility for the advance on TuneCore's site: https://support.tunecore.com/hc/en-us/articles/115006695368-TuneCore-Direct-Advance-Frequently-Asked-Questions.
If you don't see the button in your TuneCore account, but believe you qualify for an advance, please contact TuneCore's Customer Care team here: https://support.tunecore.com/hc/en-us/articles/115006501747. They can help you determine why the button didn't show for you on the TuneCore site.