Error & how to fix

Simple XML Sitemap IndexNow
Verification key not available
An IndexNow verification key needs to be generated and optionally added to settings.php in order for IndexNow engines to get notified about changes. This warning only applies to the production environment.

Go to: /admin/config/search/simplesitemap/search engines/settings

Generate verification key

Go to web/sites/default/files

add folder name private (if not exist)

create .htaccess and put this line

# Deny all requests from Apache 2.4+.
<IfModule mod_authz_core.c>
 Require all denied
</IfModule>

# Deny all requests from Apache 2.0-2.2.
<IfModule !mod_authz_core.c>
 Deny from all
</IfModule>

   # Turn off all options we don't need.
   Options -Indexes -ExecCGI -Includes -MultiViews

Full error massage:

Entity/field definitions
Mismatched entity and/or field definitions
The following changes were detected in the entity type and field definitions.
Custom menu link
The Custom menu link entity type needs to be updated.

How to fix:-