XenApp Services is a legacy protocol intended to allow legacy clients such as PNAgent to connect to Web Interface (the predecessor to StoreFront).
Citrix Workspace app used to maintain the ability to connect to a XenApp Services URL for backwards compatibility with Web Interface. StoreFront also provides a XenApp Services URL for backwards compatibility with legacy clients. However it was also possible (but not recommended) to connect Citrix Workspace app to the XenApp Services URL provided by StoreFront.
With Citrix Workspace app for Windows 2409 the ability to connect to a XenApp Services URL was removed. When users open Citrix Workspace app configured with a XenApp Services URL, they see the following error:

You must configure Citrix Workspace app to connect to the Store URL. You can view the Store URL within the StoreFront management interface:

The steps to configure Citrix Workspace app depend on the following:
GPO: If the PNAgent store was configured using GPO, exit Workspace app, remove “PNAgent” URL from GPO settings and use store discovery URL, for example https://storefront.example.com/citrix/store/discovery
Below are the steps to change the GPO:
Open the Citrix Workspace app Group Policy Object administrative template by running gpedit.msc.
Under the Computer Configuration node, go to Administrative Templates > Classic Administrative Templates (ADM) > Citrix Components > Citrix Workspace > StoreFront.
Select Citrix Gateway URL/StoreFront Accounts List.
Select the Enabled option and click Show. If you enable this policy setting, you can enter a list of StoreFront Accounts and NetScaler Gateway URL.
Enter the URL in the Value field.
Specify the store URL that is used with the Citrix Workspace app:
STOREx="storename;http[s]://servername.domain/citrix/<storename>/discovery;[On, Off]; [storedescription]"
Manual: If the PNAgent store was configured manually, then the user has to
Either manually remove the store or select ‘Reset Workspace App’
Add the storefront URL. For example https://storefront.example.com/citrix/store/discovery
Two options are available if the store was configured using the installer command line.
Option 1:
Uninstall the existing version.
Install with the store added in the cmd line as mentioned in https://docs.citrix.com/en-us/citrix-workspace-app-for-windows/install.html#examples-of-store-configuration-using-command-line-installation
Option 2:
You can use cleaninstall parameter as mentioned in https://docs.citrix.com/en-us/citrix-workspace-app-for-windows/install.html#command-to-cleanup-and-install-citrix-workspace-app to combine both the steps above
The authentication method for the store URL may be configured differnetly than the XenApp Services URL in StoreFront. With the StoreFront management console, click "Manage authentication methods" and ensure that the applicable authentication method is selected.
If the Store URL is HTTP, it is strongly recommended that you configure StoreFront for HTTPS. By default, Citrix Workspace app will not connect over HTTP. If you must use HTTP then follow the steps mentioned in the article - https://support.citrix.com/s/article/CTX134341-how-to-configure-citrix-receiver-for-windows-to-manually-add-http-stores?language=en_US
If you are using the domain pass-through auth method, you need to add the store URL to an intranet or trusted zones as mentioned in https://docs.citrix.com/en-us/citrix-workspace-app-for-windows/authentication#modify-the-internet-explorer-settings
If you are using the command line, you need to add the store URL as mentioned in https://support.citrix.com/external/article/CTX200337/driving-the-citrix-receiver-selfservice.html
Effect on User Interface
Once Citrix Workpace app is configure to connect to the StoreFront using a store URL, it displays either the Modern user experience or the Classic user interface, depending on which is configured within StoreFront.
CWA 2403 onwards, the support for “PNAgent” has been deprecated and removed.
https://docs.citrix.com/en-us/citrix-workspace-app-for-windows/deprecation.html

Citrix Workspace app 2409 or later no longer support XenApp Service URLs (also known as PNAgent URLs, e.g. https://url/citrix/store/pnagent/config.xml). If Citrix Workspace App is configured to connect to a XenApp Services URL, you will see error message on refresh or sign-in stating, “To resolve this issue, contact your helpdesk with this information: Error while parsing.” You must reconfigure Citrix Workspace app to use a store URL.