Update: I just realized that I can install it if I download it and then bring it in via Apps > Upload a custom app, so it's only installing it from within App Studio that gives the error.
I'm trying to work through the sample app at https://docs.microsoft.com/en-us/microsoftteams/platform/get-started/get-started-dotnet-app-studio, but I'm stuck at the part where you install it into Teams.
I get the error: "Uploading custom apps is not allowed."
According to Prepare your Office 365 tenant, in the admin center, under Settings > Services & add-ins > Microsoft Teams > Tenant-wide settings > Apps, I should turn on "Allow sideloading of external apps." In my tenant, that is indeed turned off:
But I can't turn it on. It's disabled, saying the settings have moved to the "Microsoft Teams & Skype for Business admin center."
In that admin center, I went to Teams apps > Permission policies. There, for "Global (Org-wide default), I have "allow all apps" for everything: .
For "Org-wide app settings", I have interaction with custom apps turned on: .
Under Setup policies > Global (Org-wide default), I have "Allow uploading custom apps" on: .
And I'm signed in as a user governed by that policy:
I'm stumped. What else do I need to do so that I can sideload a Teams app for development?