FlexClip provides 3 plans for you: Free, Plus, and Business. Every plan has its limits. You can check the details on the Pricing page.
Due to copyright protection, the watermark on the free version cannot be removed.
If your video elements have exceeded your Plus plan limits, a watermark will also appear on your project.
Click the Export button to check which limit you have exceeded. Re-edit your video project following the instructions, and then you can remove the watermark successfully.
Here is an example for your reference:
Also read: What does stock video mean?