SFDMU Help CenterThe GUI App is failing to connect to the salesforce org.
Symptom.
If you are getting an error after selecting the source and the target Orgs and clicking the "Next" button, you probably has a general problem connecting to you Org using the selected SFDX org alias.
Solution.
First, check the org connection by running the CLI the force:org:display command in your Terminal / Console and make sure that the connection is not expired and you have a clean console output.
Below is an example of incorrect command output:
This is correct command output:
Another possible reason is that you might have inaccessible orgs stored in the local SFDX vault.
For the solution, refer this article: