I see you’ve tried to set up CNAME verification for your domain, however it has been done wrongly.
You need to point 6557cc10588adc57942d79c5f9957d71.testing.cloudns.be to ns1.byet.org, however your current configuration is pointing ns1.byet.org.testings.cloudns.be to pointing6557cc10588adc57942d79c5f9957d71.testings.cloudns.be.
Fixing that should allow you to add your domain.