If you get the message when trying to add a reviewer to your workflow, please ensure the spreadsheet owner has not prevented you from adding new people.
Form Workflow Plus will automatically add reviewers as editors on the spreadsheet. If the spreadsheet owner has prevented editors from adding new people you will not be able to add reviewer(s).
User email@example.com creates a Google Sheet and shares it with the company's workflow creator, firstname.lastname@example.org, but in the process prevented other editors from adding other people to the sheet.
When user email@example.com opens the sheet and activates Form Workflow Plus to specify conditional approvals/add reviewers, the following error would come up.
To resolve this, the spreadsheet owner would simply need to update the settings making sure editors can change access and add new people to the sheet.