1.  Open Web.config file located in the DotNetNuke root folder

2.  Locate line with “<assemblyBinding”

3.  Add the following immediately below that line

<dependentAssembly>
    <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly>
<dependentAssembly>
    <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly

4.  Save changes and try accessing the application again

Reference:  HOW TO FIX: "requires a ScriptManager on the page" Error when using .Net 3.5