I went to your App page on the App Store, i think your "Marketing URL" is empty. You'll need to re-publish with Marketing URL filled.

They use Marketing URL to reach the app-ads.txt

Step 1a https://support.google.com/admob/answer/9363762?sjid=988135710817718778-NA#step1a

For Apple App Store: Add your developer website in the marketing URL field of your store listing. Check if you can see the "Developer Website" link displayed at the bottom of your iOS app page to confirm that you successfully added the URL.