If you placed the verification code, ads.txt, and ad tags correctly - the ads will start to display within 24 hours.

If the ads do not appear make sure that your ads.txt, verification, and tags are placed according to the instructions in your approval email. 

Subdomain vs. Top-domain.
Make sure to keep your ads.txt file on the top domain and not the 'www' subdomain. By default, crawlers use the top-domain, and won't fetch subdomain ads.txt unless redirected. Correct: https://domain.com/ads.txt. Incorrect: https://www.domain.com/ads.txt.

Most common issues related to the codes/tags being placed incorrectly.

Do not copy tags/codes from the approval email, download the files and copy from files.

Create a separate test page or pure HTML and place the codes there, this will ensure that none of other scripts/plugins do not interfere with the ads.

If the ads are still not working please check if you have any caching/speed plugins (WP Rocket) that could be minifying scripts.

If you tried all of the above and the ads still not working - submit a ticket to support.