Could not resolve <s:Application> to a component implementation

[ShaningYu]

I opened a project that was done by a Developer before. For the index.mxml, it shows such an error:
Could not resolve <s:Application> to a component implementation on Line 10 (in red below):
<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:shp="com.esri.shp.*"
xmlns:mx="library://ns.adobe.com/flex/mx"
minWidth="955" minHeight="600"
initialize="initCollections();"
xmlns:esri="http://www.esri.com/2008/ags"
applicationComplete="application1_applicationCompleteHandler(event)"
creationComplete="application1_creationCompleteHandler(event)" >
......
protected function application1_creationCompleteHandler(event:FlexEvent):void {
addEventListener(KeyboardEvent.KEY_DOWN, deleteKeyHandler);
}

I could not figure out what kind of problem it is. Can anyone provide a hint? Thanks.

[ShaningYu · Accepted solution]

Problem solved as I re-set to use API 4.51 to substitute 3.5B and use Player 10.2 to substitute 10.0.