Right click on the solution and go to "manage nuget packages" and make sure that Microsoft.SharePointOnline.CSOM version 16.1.6621.1200 is installed as a nuget. If it is then you need to right click on the solution and do "restore nuget packages." If it's not you need to select that version of the nuget and install it in the solution. Then you should see it in the bin folder after you run build. If it's not in bin you need to find where the nuget package is installed and grab the dll manually to put in the bin folder OR add a before build task to copy the dll from your lib folder into the bin upon build.
https://stackoverflow.com/questions/49361618/microsoft-sharepoint-client-runtime-portable-is-missing