Sideloading of apps is not enabled on this site.

This error occurs when we want to deploy an app to the site other than a developer site.

Steps to enable Sideloading
Install SharePoint Online Management Shell from here.
Change the site url user name and password highlighted in red.
run the script through SharePoint Online Management Shell.



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

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




    $siteurl = 'OFFICE365 SITE URL'
    $username = 'USER ID'
    $password = ConvertTo-SecureString -String 'PASSWORD'`
                -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();
}