You are using the .htaccess Maker to create a security optimized .htaccess file. The Front-end Protection option, enabled by default, disables direct web access to all files (except Joomla's index.php file and select file types in the media, components, modules and plugin folders) that you do not explicitly allow. A .txt file in the root of your site would, therefore, be blocked with a 403 Forbidden error just like you describe.
As I said, you can explicitly tell it which files to allow. Go to Components, Admin Tools, .htaccess Maker and add the following to the "Allow direct access to these files", in a new line:
ads.txt
Then click on
Save & Create .htaccess to apply your changes.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!