This indicates that there is a version mismatch or missing assembly reference related to the RestSharp library in your UiPath project.
Try with these troubleshooting steps
Ensure that you are using the correct version of the RestSharp library in your project. The error message specifies “Version=106.6.10.0,” so make sure you have this exact version referenced.
Remove RestSharp and UiPath.WebAPI.Activities from your project, then reinstall them using the Package Manager to ensure a clean installation.
Ensure that the RestSharp library you are using is compatible with the .NET Framework version that your UiPath project is targeting.
Or
Sometimes, restarting UiPath Studio can help resolve reference issues.
Check this out as well
Cheers @Chakravarthi_Koppuravuri