Convert User ID to Name with AMPScript

I am trying to populate the name of the contact owner (‘referred to as the retention owner in our organization) in an email subject line in Marketing Cloud using AMP Script.

Based on the code below, I am able to bring over the ID but am wanting to convert the ID to the individual’s name. Below is what I have so far.

%%[
  VAR @retentionOwner
  SET @retentionOwner = AttributeValue("Retention_Owner__c")
]%%
%%=v(@retentionOwner)=%%

May I have advice and guidance on what I am missing?

Thanks

%%[ 
VAR @retentionOwner 
SET @retentionOwner = AttributeValue("Retention_Owner__c") 
SET @ownerName = Lookup(User_Salesforce,”Name”,”Id”, @retentionOwner)
]%% 
%%=v(@ownerName)=%%