Before troubleshooting activation, identify the exact app and where you obtained it. The App Store, direct-download and Setapp editions use different licensing paths. iFlicks and the one-time-purchase iFlicks 3 edition also need to be distinguished.
App Store
Sign in to the Mac App Store with the Apple Account used for the purchase. To reinstall, open your purchased-app list and download the same app. For the subscription edition, use Restore Purchased Subscriptions… in iFlicks where available. A one-time app purchase and an in-app subscription are different purchases.
Direct download
Use the download and license or subscription details associated with your original order. Follow the activation or restore controls offered by that edition. If a purchase is not recognized, contact support with the purchase channel and order reference rather than buying again as a troubleshooting step.
Setapp
Install or open the Setapp edition through the Setapp account providing access to it. An entitlement from another store should not be assumed to activate that edition, or vice versa.
A feature is unavailable
In the subscription offering, Basic covers core metadata and video processing; Plus adds advanced features such as rules, watch folders, custom presets and track-detail editing. Check the options shown in your installed edition before changing a plan. Current availability and pricing are shown in the relevant purchase channel.
For help, send the app name/version, source, exact activation message and order reference if available. Never send a password or full payment-card details. Support can investigate a purchase; this guide does not establish a transfer, refund or upgrade entitlement.
Reference: redownloading App Store apps and iFlicks subscription information.