Changing placeholder texts in Pages using AppleScript
set … to every … sets the variable to a list. That means thistext is a list, so you can't perform operations meant for the items in the to the list itself. You need a loop:

repeat with thistext in (every text item whose object text contains "Footnotes")
    …
end repeat


The full script is:

tell application "Pages"
    activate
    tell document "test"
        tell current page
            repeat with thistext in (every text item whose object text contains "Footnotes")
                tell thistext
                    tell its object text
                        set every word to mytext
                    end tell
                end tell
            end repeat
        end tell
    end tell
end tell