Is it not possible to create new pages under a linked page ?

Your colleague already asked this question here. As you can see from the message you are getting it is not possible. It makes no sense and you should consider changing the tree structure. Keep in mind what is the linked page's design - there is still only one record in CMS_Tree table which ensures the tree structure. This is to prevent loops in the tree structure.

Hi Juraj Ondrus,

I know this is loops in the tree structure and only one record in the CMS_Tree table. But I need the Products page to create child links pages under it and before it was possible to add link pages under the linked page in Kentico 11. So I trying to implement the same in 13.

What I ended up doing in this scenario, is to create a new page type "Linked ____" that has a NodeGuid reference to the page you want to link to, then add logic that "if it's a linked page type, get the reference nodeguid and switch it with the thing it's linking"

Not ideal, but linked documents are starting to go away and may want to avoid using them.

You can also use Related Pages instead of parent-child relationships, so you can have multiple pages reference the same items.