Hello, the reason for this is that, well, your file size exceeds 20 Megabytes. (No, it's not MecaBricks) In other words, your model is too big to render for free.
You can pay for a larger render by buying credits here: https://www.mecabricks.com/en/credits
Obviously, not everyone can afford to buy renderings, and it's great that Scrubs provides small ones for free! However, he really needs the money from the larger renders to keep the website alive, and the free renders actually aren't free, Scrubs pays for them.

There is a way to render any size of model, any size of picture, for free still: You can download Blender(a free 3D design and rendering program) here: https://www.blender.org then you can get the free add-on for blender that enables you to render Mecabricks models here: https://www.mecabricks.com/en/shop/product/2 You may want to watch a simple tutorial on the Blender user interface as it can be difficult for beginners, but there are many of these on youtube. Then simply follow the instructions in the PDF that comes with the Blender Lite download, and render your model! Keep in mind this will take much longer than on the renderfarm.

Hope this helps,
ArmoredBricks