How to fix SharePoint 2013 workflow missing issue on SharePoint online site?

If users could not find the out of box SharePoint 2013 workflow on SharePoint online site, here are two different ways to verify and fix the issue.

The first way is to click "Setting"->"Workflow Settings"->"Work Flows"->"Workflow Health" and then activate "The workflow service store feature" as described here.

If you would to fix on multiple site collections, here are the PowerShell script you can run against the site.

1. Install SharePoint online PnP PowerShell package.

Install-Module SharePointPnPPowerShellOnline

2. Check the WorkflowServiceStore site collection feature

Connect-PnPOnline -Url <Site URL>
Get-PnPFeature -Identity 2c63df2b-ceab-42c6-aeff-b3968162d4b1

3. Enable the WorkflowServiceStore site collection feature

Enable-PnPFeature -Identity 2c63df2b-ceab-42c6-aeff-b3968162d4b1 -Force

That's it.