Thanks for using Syncfusion product.
We have analyzed the reported issue in our end, we found the assemblies which referred only the 4.6 version. This error occurred due to the mismatching of frame work version and Syncfusion assembly reference version. For this we suggest you to change the version of the application frame work in project file to 4.0 or use assembly reference as 15.4400.0.20. Otherwise you have to refer the dll’s that are installed in the system and the assembly details can be found from the following installed location.
“<Installed Location>\Syncfusion\Essential Studio\126.96.36.199\precompiledassemblies”
You can refer the below KB link to avoid the assembly conflicts,
Also you can refer the below forum which provides more details about the same issue which you are faced.
Let us know whether this helps also if you need any further assistance on this.