I’ve followed the instructions here but it tells me
Your PHP code changes were not applied due to an error on line 1 of file wp-content/plugins/mainwp-custom-dashboard-extension/class/mainwp-custom-dashboard.class.php(68) : eval()'d code. Please fix and try saving again.
Cannot redeclare admin_default_page() (previously declared in wp-content/themes/hello-elementor/functions.php:232)
This file doesn’t have 232 lines in the latest version, so it looks like you added the code at yourself at the end of that file. You shouldn’t, because it gets overwritten with the next update.
Remove it there and put the code in the code snippet in the Custom Dashboard Extension as instructed. Then it should work.