To add an administrator to a classic site, follow these steps.

1. Download and install the latest version of the SharePoint Online Management Shell from https://go.microsoft.com/fwlink/p/?LinkId=255251.
2. Launch PowerShell and import the SharePoint Online module:

Import-Module SharePointOnline

3. Once it has been imported, you'll need to connect to SharePoint Online:

Connect-SPOService -Credential (Get-Credential) -Url https://<tenant>-admin.sharepoint.com

4. Set the user as a Site Collection Administrator:

Set-SPOUser -Site https://<tenant>.sharepoint.com/sites/<site> -LoginName <[email protected]> -IsSiteCollectionAdmin $true

5. If you want to promote the user to the primary Site Collection Administrator, perform the following step:

Set-SPOSite -Identity https://<tenant>.sharepoint.com/sites/<site> -Owner <[email protected]>