Why AWS Organization doesn't allows me to add more accounts?

Hi there. I have recently started to study AWS in order to get a certification. I'm studying the AWS Organizations subjects, I created one and added an account, but when I try to add a 3th (using the AWS Organization web console) account I get the error:

Ok, I created the 3th account manually (the usual way to create an AWS account), in that account I created an user and give him AdministratorAccess permission. Returned back into the main account, go into the AWS Organizations web console and tried to invite the 3th account into my organization but I get a similar error:

I know that there is a limit by default (as stated in https://docs.aws.amazon.com/organizations/latest/userguide/orgs_reference_limits.html#min-max-values) which is 10, but I don't know the why if the limit is 10 I can't add more accounts.

Also, I requested in the AWS Service Quota web console to increase the limit, but it has been more than 12h and my case is still not Assigned to anyone :-(

So the question is: why is there a limit which by default I can't add more accounts? what is the reason to that limit?

Greetings :-)