I'm not sure you are putting the expression in correctly. You have to add it over in the dynamic content box and add it in the field. You can't just type the expression into the field.
Here is a quick example email I made:
Example Email Body
My email message body looks like this:
Here is a bunch of website data from somewhere in outer space.
First Name: Steve
Last Name: Harvey
City: New York
Province: Canada
Message: Hey welcome to family feud!
Here is the expression to get the First Name:
trim(first(split(last(split(outputs('Html_to_text')?['body'],'First Name:')),'Last Name:')))
Here is the expression to get the Last Name:
trim(first(split(last(split(outputs('Html_to_text')?['body'],'Last Name:')),'City:')))
Here is a screenshot of the flow:
Flow Example - First Name and Last Name
This is the expression to get the City:
trim(first(split(last(split(outputs('Html_to_text')?['body'],'City:')),'Province:')))
This is the expression to get the Province:
trim(first(split(last(split(outputs('Html_to_text')?['body'],'Province:')),'Message:')))
This is the expression to get the last row Message:
trim(last(split(outputs('Html_to_text')?['body'],'Message:')))
Here is a screenshot of the flow and where the expression is added or updated:
Flow Example 2 - Expression
Can you use this as an example and get the values you need?