I tried to add and verify my domain as a secondary domain but it's not working many times

[AhmedHassanSE]

I tried to add and verify my domain as a secondary domain but it's not working many times and I removed it and try to add to a new email account but It give me message that this domain is already exists and I cannot add it again to the first account as a secondary domain 

I need to add and verify my domain as soon as possible