Cannot mix 2018 and 2021 terms in call to TextTheme() constructor

@mister_cool_beans is correct. Flutter is transitioning to Material 3 text style names for TextTheme. We are in the awkward period where both sets exist. See more here for the commit associated with the change.

To be more clear, you must only use terms from the 2018 set or from the 2021 set. Update your TextTheme/ThemeData correspondingly. I suggest you take the opportunity to upgrade to 2021.

2021 set

displayLarge, displayMedium, displaySmall 
headlineMedium, headlineSmall 
titleLarge, titleMedium, titleSmall 
bodyLarge, bodyMedium, bodySmall 
labelLarge, labelSmall

2018 set

headline1, headline2, headline3, headline4, headline5, headline6
subtitle1, subtitle2
bodyText1, bodyText2
caption, button, overline

As @Partick mentioned, if you scroll down to the implementation seen here you can see the sets defined in code.

...
}) : assert(
       (displayLarge == null && displayMedium == null && displaySmall == null && headlineMedium == null &&
           headlineSmall == null && titleLarge == null && titleMedium == null && titleSmall == null &&
           bodyLarge == null && bodyMedium == null && bodySmall == null && labelLarge == null && labelSmall == null) ||
       (headline1 == null && headline2 == null && headline3 == null && headline4 == null &&
           headline5 == null && headline6 == null && subtitle1 == null && subtitle2 == null &&
           bodyText1 == null && bodyText2 == null && caption == null && button == null && overline == null),
       'Cannot mix 2018 and 2021 terms in call to TextTheme() constructor.'
     ),
...

Please note that you can not mix and match the 2018 styles with the 2021 styles. Only one or the other is allowed in this constructor. The 2018 styles will be deprecated and removed eventually. Source: Source Document