Hi Muhammad Refaat,
Thank you for contacting Syncfusion support.
We suspect this issue may occurs due to following two reason.
1. 'Syncfusion.DocIO.Base' and one of its dependencies assembly version is mismatched in web.config. So kindly ensure that you have provided the version as same as referred version in your project.
2. 'Syncfusion.DocIO.Base' and one of its dependencies is not present in specified location. Please ensure DocIO assembly is present in specified location.
So, for your convenience we have created a ASP.NET MVC for document editor by referring all our dependencies as Nuget packages . Please find the sample from following link
Kindly try this sample and let us know if this helps you.