Hey David,

I've checked your site and found that you have used wrong installer to upgrade PayPlans at your site.
That's why you are hitting this error and component gone.

I've already helped you to upgrade PayPlans to version 3.7.1 version your site. Please take a look again on this and update us if still issue persists.

Before upgrading to PayPlans 4.0 , please refer PayPlans transition doc. If you want us to upgrade PayPlans 4.0 at your site , then create backup of your site first and update us , so we will help you to upgrade.
https://stackideas.com/docs/payplans/administrators/welcome/transition-to-payplans-4

Update us if you have any query.

Thank you for understanding !