To enable SideLoading feature in Non - Developer Site for App Deployment

Add-PSSnapin Microsoft.SharePoint.PowerShell

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

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

Write-Host `
  'To enable SharePoint app sideLoading, ' + `
  'enter Site Url, username and password'

$siteurl = Read-Host 'Site Url'
$username = Read-Host "User Name"
$password = Read-Host -AsSecureString 'Password'

if ($siteurl -eq '') {
    $siteurl = 'https://vigtech.sharepoint.com/sites/MyTeamSite'
    $username = 'sivavicky@vigtech.onmicrosoft.com'
    $password = ConvertTo-SecureString -String 'mypassword!'`
                -AsPlainText -Force
}
$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
    $sideLoadingEnabled = `
    [Microsoft.SharePoint.Client.appcatalog]::IsAppSideloadingEnabled($cc);
   
    $cc.ExecuteQuery()
   
    if($sideLoadingEnabled.value -eq $false) {
        Write-Host -ForegroundColor Yellow `
          'SideLoading feature is not enabled on the site:' $siteurl
        $site = $cc.Site;
            $sideLoadingGuid = `
           new-object System.Guid "AE3A1339-61F5-4f8f-81A7-ABD2DA956A7D"
            $site.Features.Add($sideLoadingGuid, $false, `
            [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None);
            $cc.ExecuteQuery();
           Write-Host -ForegroundColor Green `
          'SideLoading feature enabled on site' $siteurl
    }
   
    Else {
        Write-Host -ForegroundColor Green `
          'SideLoading feature is already enabled on site' $siteurl
    }
}

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