Hi @MKMUMU :
Could you tell me what your data source is? Share Point? CDS?
Do you want to know how to get the email address of the creator of a record?
Do you want to know how to send that user an email if the Task Status is changed from "New" to "Completed"?
I assume your data source is Share Point list.( It has a system field "Created by", which records the information of the creator of the record)
I’ve made a test for your reference:
Here is my data source: List B
1\Add Office 365 Outlook as your data source.
View-Data sources-Connectors-Office 365 Outlook-Add a connection

2\ Put this code after your code which is used to change the task status:
If(
First(
Sort(
'List B',
Modified,
Descending
)
).'Task Status' = "Completed",/* Find the most recently modified item */
Office365Outlook.SendEmailV2(
First(
Sort(
'List B',
Modified,
Descending
)
).'Created By'.Email,/*Get the user’s email address*/
"Subject",/*the email’s subject*/
"Body"/*the email’s body*/
)
)
Best Regards,
Bof