The remote server returned an error: (401) Unauthorized when importing a form template

  Issue

When you attempt to import a form template you receive the following error.

401 Unauthorized

  Resolution

It appears that this issue can occur if your tenant was created after August 2020 and it’s quite easy to resolve by running a single PowerShell command. You simply connect to SPO using the SharePoint Online Management Shell and then run the command. It can take a few minutes to take effect, in our case it took less than 5 minutes.

Connect-SPOService -Url https://yourtenant-admin.sharepoint.com
Set-SPOTenant -DisableCustomAppAuthentication $false

Please note that “yourtenant” will be the word before .sharepoint.com in the URL of your SharePoint site. Press the Enter key after each line to run the commands.

Set-SPOTenant DisableCustomAppAuthentication $false

Source: https://docs.microsoft.com/en-us/answers/questions/90187/sharepoint-app-only-add-ins-throwing-401-unauthori.html