Hi Friends,

 I found this problem during my first use of ASP.Net AJaX toolkit. To resolve this problem you have to add the validation.dll file in your bin folder and just commnet your tagMapping block exist in the Web.Config file and add the following:    

<tagMapping>
<add tagType=”System.Web.UI.WebControls.CompareValidator” mappedTagType=”Sample.Web.UI.Compatibility.CompareValidator, Validators, Version=1.0.0.0″/>
<add tagType=”System.Web.UI.WebControls.CustomValidator” mappedTagType=”Sample.Web.UI.Compatibility.CustomValidator, Validators, Version=1.0.0.0″/>
<add tagType=”System.Web.UI.WebControls.RangeValidator” mappedTagType=”Sample.Web.UI.Compatibility.RangeValidator, Validators, Version=1.0.0.0″/>
<add tagType=”System.Web.UI.WebControls.RegularExpressionValidator” mappedTagType=”Sample.Web.UI.Compatibility.RegularExpressionValidator, Validators, Version=1.0.0.0″/>
<add tagType=”System.Web.UI.WebControls.RequiredFieldValidator” mappedTagType=”Sample.Web.UI.Compatibility.RequiredFieldValidator, Validators, Version=1.0.0.0″/>
<add tagType=”System.Web.UI.WebControls.ValidationSummary” mappedTagType=”Sample.Web.UI.Compatibility.ValidationSummary, Validators, Version=1.0.0.0″/>
</tagMapping>  

Hope it will resolve your problem.    
Regards
VBM