Advanced Ads creates the ads.txt file dynamically and you won’t find it as a physical file on your server.
You can find the ads.txt options in Advanced Ads > Settings >General, where you already enabled the ads.txt. I guess that you will see there is something like:
The following line will be added automatically because you connected your AdSense account with Advanced Ads: google.com, pub-12300123001230213, DIRECT, f08c47fec0942fa0
Adsense is currently the only entry in your ads.txt. If you want to add more entries, just put them into the related code field in the ads.txt options.
You can find more detailed information in our ads.txt manual and this video:
