Have you encountered this issue in SXA?
To re-create the issue, In the Experience Editor Click Presentation Details > Click Final Layout tab > click Edit > click Placeholder Settings > click Add
I tried installing a fresh Sitecore 9 instance then installed SXA 1.6 and Sitecore PowerShell Extension 4.7.2 and I encounter the same error. Anyone has info on how to resolve this issue?