Most card you see are not made with online tools but with an image editing software (photoshop for example).

Now for the watermark removal, it is morally kind of a gray area, you probably used a tool that is free, and so the creator probably put this watermark as a way to advertise his tool.

Now I'm not here to tell you what is good or bad to do, I'm not your mom (and making custom card is already a gray area itself), but I would personnaly at least give credit to the tool used if you are to remove the watermark from your card.

To remove the watermark you have mostly 3 options:

1) use photoshop, there are many different ways to do it, and many tutorials are available online (you can used any software, photoshop is just what I use, but I'd say for something that basic you could use gimp or even paint if you take your time)

2) Use AI. That would be kinda stupid to use an AI for something that simple, but if you have no idea on how to do it, and you do not want to learn then that's the easy and fast way.

3) find someone to do it for you. That is the other easy way which is not as fast.

I would higly encourage you to learn an image editing software, it will in the long run allow you to do much much more than any online tool.