I have created a community page from a free developer org, which is not allowing me to assign knowledge articles to topics under the "Navigational Topics" tab. When I select the article and try to save, it returns the message "Assigning the topics you've selected isn't allowed. A Salesforce admin must enable topics for objects for the article type(s)."

In my org I am a system administrator. What could be wrong?

Hi,
I am not able to understand the below question correctly Please help me out. - 
Create a class that has a method accepting two strings. The method searches for contacts that have a last name matching the first string and a mailing postal code (API name: MailingPostalCode) matching the second. It gets the ID and Name of those contacts and returns them.The Apex class must be called 'ContactSearch' and be in the public scope.
The Apex class must have a public static method called 'searchForContacts'.
The 'searchForContacts' method must accept two incoming strings as parameters, find any contact that has a last name matching the first, and mailing postal code matching the second string. The method should return a list of Contact records with at least the ID and Name fields.
The return type for 'searchForContacts' must be 'List<Contact>'.

Project custom object records should only be seen by the owner of the record and users above the owner on the role hierarchy. However, for some Project records, the Training Coordinator must also have Read Only visibility to the Project record.

Create a custom picklist field on the Project object called “Priority” with the following values: High, Medium, and Low. Then create a criteria-based Sharing Rule for Project records where the Priority = High to share those records with the Training Coordinator role.