There are many files that crawlers expect to find in well-known locations on websites, one such file is ads.txt. While you might not have paid advertisements, crawlers may still look for a copy of this file leading to HTTP 404 errors in your logs. To prevent the error and show that you should have no advertisements leading there you can add the file with placeholder values as follows:
In the root of your website, create a new file with the name ads.txt.
#ads.txt - no DIRECT or RESELLER www.example.com, placeholder, DIRECT, placeholder # NONE
NOTE: If you ever do use an advertiser, they will generally inform you as to changes to make to this file.