As of September 2026, the SharePoint Subscription Edition CU, the ToolPane used for editing SharePoint Web Parts has been disabled. This means that every time a CU is installed you must re-enable the ToolPane in order to edit Web Part. It might be a good thing to keep this disabled unless it's required.

To enable it again open SharePoint Management Shell As Administrator and execute this script.

Get-SPSite -Limit All | ForEach-Object {
    Write-Host "Enabling ToolPane: $($_.Url)" -ForegroundColor Cyan
    Enable-SPToolPane -Identity $_.Url
}

To disable it run this script.

Get-SPSite -Limit All | ForEach-Object {
    Write-Host "Enabling ToolPane: $($_.Url)" -ForegroundColor Cyan
    Enable-SPToolPane -Identity $_.Url
}

You might see this error

Check SharePoint SSE September 2026 CU Notes.

 https://support.microsoft.com/en-us/servicing/office/hotfix/sharepoint/5002908