I was trying to complete the Postman 30 day challenge but the submission requires a public workspace. I tried changing the visibility of my workspace to public but it showed me the following error.
If youโre in a hurry, it maybe quicker to create a new public workspace, export all the content of the original workspace to files, and then import those files into the new public workspace.
I tried creating a new public workspace but it showed the same error and created a workspace with the same details with the visibility set to team.
You will need to ask a Community Manager in your organization to change this / create a new workspace with public visibility for you. It seems only Community Managers can do this in enterprise / professional plan scenarios:
Taken from Defining roles
Ok. I will ask around in my organization. Thanks a lot for your help.
Hi! Like @xavier.gutierrez mentioned, if you are part of a team only the Community Manager of your team can approve your workspace for public visibility. Were you able to get in touch with your Community Manager?
This is why the instructions in Day 00 say to make your workspace public from the start, so you donโt end up doing all the work in a private workspace.
While getting your Community Manager to approve the public workspace is the easiest solution, if you are unable to get a hold of them or donโt want to wait you can copy all your collections to a new workspace that you own in a separate personal Postman account. Of course, make sure your workspace is public 
You can either export/import each collection one by one from your completed workspace, or try this tool developed by a Postmanaut for copying all collections from one workspace to another: Workspace copy tool
I have created a new public workspace in a separate postman account and have submitted the pull request from there. Thanks a lot for you help.