Hi Humanjo Daro,
Starting with version 17.1.0.32 (2019 Volume 1) Syncfusion will no longer publish the NuGet packages in nuget.syncfusion.com. All Syncfusion Essential Studio components are available in NuGet.org from version 16.2.0.46 (2018 Vol 2 Service Pack 1). So, if you are using the nuget.syncfusion.com feed link to install/update Syncfusion NuGet packages the error will be occurred since the latest Syncfusion NuGet packages are not available in the Syncfusion NuGet feed link, so remove the Syncfusion NuGet feed link from NuGet.config using NuGet Package Manager Settings.
Please follow the steps in the below link to migrate the nuget.syncfusion.com to nuget.org from below link,
If you are still facing the issue, please check your NuGet.Config file have other unsupported NuGet feed links. If other unsupported NuGet feed links are configured, please remove all the unsupported NuGet feed links from NuGet.Config file and try to install/update the Syncfusion NuGet packages from nuget.org.
Regards,
Ganesan Rengasamy.