App support (how-to instructions and troubleshooting) will have to come from the developer.
According to their website, any subscription tier beyond the free trial is meant to remove the WeVideo watermark.
Note that subscriptions from in-app purchases on Apple devices are for that Apple ID account only. Non-Apple devices or other accounts can’t use those subscription perks.
On what device is this happening? Did you subscribe on that same device?
Subscriptions that you do not wish to continue, should be canceled. If you want to cancel a subscription from Apple - Apple Support
If you want a refund, then you can contact Apple about that. Request a refund for apps or content that you bought from Apple - Apple Support