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Assuming that you have a valid username/pw to connect to Alfresco EE Repository, you need to configure your maven settings.xml with those credentials to connect to "activiti-enterprise-releases" repo which is specified in the pom.xml (activiti-custom-reports/pom.xml at master · Alfresco/activiti-custom-reports · GitHub )
If you don't have a username/pw, please raise a case with Alfresco support to generate one for you.
Regards,
Ciju