FlexClip provides 3 plans for you: free, plus, and business and every plan has its limits. You can check the details at https://www.flexclip.com/pricing.html.
If the elements in your video project have exceeded the limits of your current plan, it will ask you to upgrade the plan to download this video. Or you can choose to download your video with a watermark.
Please click the Export button, then you can check which limit is exceeded. Please re-edit your video following the terms, then you can remove the watermark/export your video successfully.
Here is an example for your reference: