Enable Sideloading of apps

Prerequisites :

Site should be created using developer template.

2-Steps Solution:
1) Download and install SharePoint Online Management Shell
2) Execute the script below. (Note that the Sideloading Feature ID is harcoded in SharePoint, as are many other OOTB features)

#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

In your SPO Management Shell enter the url of the site you want to deploy the solution, the administrator username and the password. The powershell will take care of the rest.