Unable to set preserve order as true in TestNG xml file

[akash mahato]

I am trying to execute testcases using testng.xml. But for some reason the classes present inside testng.xml are not executing sequentially.It is executing alphabetically. I have given preserve-order as true. But it is showing below mentioned issue while setting it as true.

redundant default attribute value assignment

I am using intellij as ide. Please find the attachment for better understanding