Taboola and Outbrain will only approve and accept high quality websites that have lots of high quality content on them and the traffic to the website should be around 100,000 monthly unique visitors minimum before you'll get approved.
A website with less than 100,000 monthly unique visitors will make some money but not much anyway.
If your website is getting less than 100,000 monthly unique visitors then try media.net or Adsense since those work with less website traffic.
Content.ad is another alternative to Adsense, Media.net and Taboola and Outbrain.