I am getting an error while running the test class , Leads are not getting created in the system ..

Test class snipet :

  Lead testLead = Class_Test.createLead('Test Lead', 'Test Company', '02s30000000000dAAB', 'Core');
    testLead.OwnerId = user.Id;
    insert testLead;

    system.assertEquals(testLead.OwnerId, uSBS.Id);  

 ---------------------

Class_Test :

public static Lead createLead(String lastName, String company, Id divId, String status){
   
    Datetime t = Datetime.now();
    Date da = Date.newInstance(t.year(), t.month(), t.day());
    Lead lead = new Lead(LastName = lastName, Company = company, Division = divId, Status = status, Phone = '1234', Start_Date__c = da);
   
    return lead;
  } 

Error :

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, EZAssign.CS_Lead: execution of AfterInsert

caused by: System.DmlException: Upsert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, EZAssign.CS_UserInfo: execution of AfterInsert

caused by: System.DmlException: Update failed. First exception on row 0 with id 005e00000012MJJAA2; first error: MIXED_DML_OPERATION, DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): User, original object: Lead: []

(EZAssign)
: []

(EZAssign)

: []

Any idea how to rectify this error