Try with below formula.
AND( OR(ISPICKVAL( Inbound_vs_Self_Generated__c , "SDR Generated") , ISPICKVAL( Inbound_vs_Self_Generated__c , "SDR/BDM Collaboration")), ISPICKVAL( Sales_Development_Rep__c , "")) OR operator checks the Inbound_vs_Self_Generated__c field have "SDR Generated"/"SDR/BDM Collaboration"
AND operator check both(Inbound_vs_Self_Generated__c field should have anyone value and the same time it will check the Sales_Development_Rep__c should not be null) condition are true the it will fire the validation