In some cases Dual Write gives you some cryptic error messages. Here are a couple of tips to get or find the errors.
If you get an error while mapping or trying to set up a new map. You can get a bit more info from using browser developer tools.
In Chrome, just right click and select Inspect.
Hopefully you get a bit more detail with the error.
If you are getting an integration error. There is a table that stores all the errors.
For more troubleshooting tips from Microsoft. Check out their docs page.