You would need to create the email templates beforehand and configure it.
Please refer to the below documentation url :
Can you confirm did you selected Email template in Workflow action edit/Create page for object you have created with approval property enabled.
I tried to do as you suggested. But it also has the same message. Please help me again
Can you please share the sample application xml file so that I can try and replicate the issue locally.
Also, can you please confirm the version of Rollbase you are using?