Created Issue: Can't use the WebPart [467]

When I try to add it to a page I get the below error:

Unable to add selected web part(s). An error has occurred.

When I try and add the line mentioned in the article, "NOTE: Add <asp:ScriptManager runat="server" /> to the master page," I get the below error in any pages I try to open.

An error occurred during the processing of . Unknown server tag 'asp:ScriptManager'.