The stitch limit for the Janome 300 is 64,000 stitches per design.
As has been commented on already, the design must be in the proper folder for the machine to access the design.
On the older machines it's also important to use the smaller capacity usb. In my experience it's not as important in the newer machines.
Use the size recommended for your machine, but I have used a 32g in my 15000 with no problems, but I don't put lots of stuff on the usb, because the more one there, the slower it will be to access.
I recently taught the beginner class and a student was working on an 11K, we couldn't get it to use a usb larger than the 8g.
I'm sure that the 300e is going to be similar.