Troubleshooting flow URLs launched from buttons and links
Key Takeaway
When a flow URL isn't working as expected, check the flow it references and the variables it sets. If the URL can't access a variable, the parameter is ignored, so the value never reaches the flow.
A custom button, custom link, or direct link can start a flow by pointing to the flow URL. The URL can also carry parameters that set the flow's input variables, and a retURL parameter that controls where users land after they click Finish. When a flow URL isn't working as expected, the cause is usually that the URL can't find the right flow, can't reach the right variable, or uses a parameter in an unsupported way.
How a flow URL starts a flow
A flow URL starts an active screen flow or autolaunched flow. The recommended format is /lightning/flow/Flow_Name. A URL without /lightning still works, but it doesn't get the benefits of running in Lightning.
You need the "Customize Application" permission to create and edit custom buttons and links. You need the "Manage Flow" permission to create, edit, and activate the flow.