Run the site on debug mode, Right click on page and goto page source.
In the script tag check the version (4_6_81).
<script src="../aspnet_client/system_web/4_6_81/crystalreportviewers13/js/crviewer/crv.js" type="text/javascript"></script>
Then goto the wwwroot folder aspnet_client directory.
C:\inetpub\wwwroot\aspnet_client\system_web
Check for folder with name as version 4_6_81, if there is not a folder create new folder and name it as same.
Then open folder 4_0_30319 or 2_0_50727 and copy all content and paste it into the created folder.
If this will not resolve your issue then reinstalled Crystal Report.