You may be seeing this error when starting OpenBazaar.
This means that the client has found your server, but it doesn't have the right username and password.
The first time you run OpenBazaar the client and server should share a username and password automatically. However, sometimes they don't sync correctly. If this happens, you'll need to manually edit a file to enter a new username and password.
Step One: Stop OpenBazaar
Close the application and shut down the server.
Step Two: Find the ob.cfg file
Each operating system has an OpenBazaar-Server folder which contains the ob.cfg file you need to edit and save.
Note that on some operating systems, these files may be hidden by default on your file explorer. If you can't locate these files, it's likely that you need to turn on the option to view hidden files.
Step Three: Edit the ob.cfg file
Open the file with a text editor.
Now you need to find a section near the bottom of the file that shows a username and password. Your text editor may look different than this image, but the text will be the same.
Note that both username and password have #s at the beginning; those must be removed. After removing the #s before USERNAME and PASSWORD enter in your own username and password as shown below.
Now you need to save this file and exit the text editor.
Step Four: Launch OpenBazaar again
Start OpenBazaar and wait for the same Authentication Failed message to show up.
This time, you scroll down and enter the username and password you put into the ob.cfg file into the username and password fields. Then save changes, and it should connect you.
If it doesn't connect, make 100% sure you removed the # from in front of username and password in the ob.cfg file and saved the file, and that you've entered the credentials properly in the fields.