Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
To get started, I need to know what type of computer you are using, a Windows XP, Vista, Windows 7, Windows 8, Windows 10 or a Mac.If its a Mac, please check the OSX version as follows, Click the apple sign at the top left of your screen, then click about this mac. And then where it says version, send that to me as a reply.
Also, what is the exact, word for word error message you get?
Ok, what ill need you to do is download and install a new copy of garmin express
Once installed, restart the computer. Then, start garmin express and when the program is on screen, connect the nuvi and turn on the nuvi.. Will it let you add the device now?
If not, take the error message down it gives you, word for word. Thank you,