I try to update a SharePoint list, with Site Owner, Site Members and Site Visitors.
If the Site Visitors field is blank I am getting 400 error and says no such a user.
The field is no mandatory, so to me the Flow should leave it blank as well!
any advise, please.
Can you post a screen shot of your Flow? That will be a big help to those trying to assist.
I should have been more specific - can you post a screen shot of you Flow in edit/design view?