SharePoint Online : Sideloading of apps is not enabled on this site

Please run the below script. Note : Use SharePoint Online Management Shell

Download and install SharePoint Online Management Shell 

#CODE STARTS HERE

$programFiles = [environment]::getfolderpath("programfiles")

add-type -Path $programFiles'\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.SharePoint.Client.dll'

Write-Host 'Ready to enable Sideloading'

$siteurl = Read-Host 'Site Url'

$username = Read-Host "User Name"

$password = Read-Host -AsSecureString 'Password'

$outfilepath = $siteurl -replace ':', '_' -replace '/', '_'

try

{

    [Microsoft.SharePoint.Client.ClientContext]$cc = New-Object Microsoft.SharePoint.Client.ClientContext($siteurl)

    [Microsoft.SharePoint.Client.SharePointOnlineCredentials]$spocreds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)

    $cc.Credentials = $spocreds

    $site = $cc.Site;

    $sideLoadingGuid = new-object System.Guid "AE3A1339-61F5-4f8f-81A7-ABD2DA956A7D"

    $site.Features.Add($sideLoadingGuid, $true, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None);

    

    $cc.ExecuteQuery();

    

    Write-Host -ForegroundColor Green 'SideLoading feature enabled on site' $siteurl

    #Activate the Developer Site feature

}

catch

{

    Write-Host -ForegroundColor Red 'Error encountered when trying to enable SideLoading feature' $siteurl, ':' $Error[0].ToString();

}

#CODE ENDS HERE