Yogesh, I split it into its own thread because it's different now that you can login. And moved it to server admin as this is more likely where this kind of question belongs anyways.
You can't create a function because you probably signed is as readonly which doesn't allow you to change anything, hence the name.
If you want, you can signin with tblwgadmin, but it's not at all supported by Tableau and you go completely at your own risk. You should be able to find the pwd for this in one of the configs such as tabsvc.yml or workgroup.yml