Hi,
the template Kodaline is available as J4 Version.
Check out this page: https://www.joomla51.com/joomla-templates/j51-kodaline
I
f you only possess the J3 version that is not compatible, you should buy the J4 version too => https://www.joomla51.com/joomla-4-templates/j51-kodaline-j4.

Hope this helps
Oliver

Thanks Olliver, I bought the J4 version via: https://joomla51.com/joomla-templates/j51-kodaline-j4/sign-up; that's the version that provoked the error.

Hold the front page... PICNIC issue. I bought both... somehow... Have now installed the correct version.

Is the site migrated from J3 to J4 or a freshly installed Joomla 4 installation?
If its a migration, pls delete the HTML folder of the /templates/j51_kodaline/ folder and re-install the J4 version (take care of overwrites, if you have some).
If the error still occurs and it is not a new J4 version but a migration than you could check if its a template problem and set Cassiopeia as default template and check if the error still occurs.
If yes and its a migration, have you checked all your plugins, modules and componentes (that are not Joomla core) if they are J4 compatible?
You could than deactivate the plugins, modules and componentes to see if they are responsible for the error. Especially the ones that are used in a menu item...
Thats what i would do

Edit:
I searched google about this error and I found this post in Joomla forum: https://forum.joomla.org/viewtopic.php?t=1000392
The reason for this error was an old overwrite in the HTML folder so thats what I wrote above: delete the templates/j51_kodaline/html folder, re-install the J4 version and try again....
Hope this helps
Oliver