When adding machines to a MCS catalog in DaaS Console, error message "Failed to create the computer account xxx due to the account name is too long(more than 15 characters)" is eventually prompted.
However, the 'NamingScheme' property of the provisioning scheme actually consists of 11 static alphabetic characters and 4 numeric characters in the tail, totally 15 characters.

Before the "account name is too long(more than 15 characters)" failure incident, there actually has been a massive adding machine attempts done by the customer. All those attempts failed upon the phase of creating virtual machines because of the Azure region CPU cores quota limit reached. Even though no new machine was successfully created out, 'StartCount' of the respective IdentityPool has been continuously increased by each of those attempts, eventually reaching 10000.
As a result, although the Azure region CPU cores quota limit has been addressed, the adding machine attempt still failed with error message "account name is too long(more than 15 characters)", i.e. 11 static alphabetic characters and 5 numeric characters.

The issue is addressed by the following steps.
1. Please increase the Azure region CPU cores quota limit in the Azure portal first.
2. Re-set 'StartCount' property of the IdentityPool associated with the affected MCS catalog to a desired value by remote powershell command to Set-AcctIdentityPool -IdentityPoolName xxx -StartCount xxx
3. Add new machines to the MCS catalog from Citrix DaaS console.

The article describes a MCS catalog adding machines failure as a joint result of 'StartCount'' property of the IdentityPool and Azure region CPU cores quota limit.