To verify, for the Dynamics GP 2018 instance that you have Web Client pointed at/using, if you go into Control Panel > Programs > Programs and Features, right-click on Microsoft Dynamics GP 2018, then click 'Change', in the options window select 'Add/Remove Features', you've verified that 'Web Client Runtime' is showing as being installed?
If so, when you go to the GP 2018 directory, there is a 'WebClient' folder containing 3 other folders and about 11 DLL files.
Also in the GP 2018 directory, there will be the 'Microsoft.Dynamics.GP.Web.Services.Runtime.Process' application file that we've mentioned and in the error?
The TenantConfiguration.config file, located, by default, at C:\Program Files\Microsoft Dynamics\GP Web Components\SessionCentral\ will show the Dynamics GP directory and files that Web Client is using.
If we uninstall the 'Web Client Runtime' feature, it will remove the 'WebClient' folder and the 'Microsoft.Dynamics.GP.Web.Services.Runtime.Process' application file from the GP directory, thus it adds it when we install that feature, which is required for Web Client to work.
If you have this feature installed but are not seeing the WebClient folder and/or 'Microsoft.Dynamics.GP.Web.Services.Runtime.Process' application file in the GP directory that Web Client is using, the Dynamics GP instance itself may be damaged.
You can try having all users log out of that GP instance, then rename it, i.e. from C:\Program Files (x86)\Microsoft Dynamics\GP2018\ for example, to be something like C:\Program Files (x86)\Microsoft Dynamics\GP2018_OLD\, then in Control Panel, right-click the GP 2018 instance and click 'Change', then in the options window, this time click on 'Repair' and run through that process.
If successful, the Repair process will completely re-create the entire GP directory, including folders and files, under the original name.
Once re-created, verify whether the WebClient folder and the 'Microsoft.Dynamics.GP.Web.Services.Runtime.Process' application file now exists or not.
If it still does not, you can try just installing another Dynamics GP 2018 instance, to include the 'Web Client Runtime' feature, then copying the WebClient folder and/or 'Microsoft.Dynamics.GP.Web.Services.Runtime.Process' application file over to the GP 2018 instance that Web Client is using, and then test Web Client.
Thank you,