Step 1: Please install the first msi from this link : https://github.com/pnp/PnP-PowerShell/releases/tag/3.9.1905.3
Step 2: After installing, run your powershell as administrator.
Step 3: Connect the root site of your site where you want to delete the subsite:
Step 3a: Connect-PnPOnline -url "https://domain.sharepoint.com/sites/classicsite"
Step 4: Remove-PnPWeb -Url subsite1
Other useful command which you may need - Connect-MsolService