When you are new here your downloading capacity is limited but your number of uploads are not restricted. But due to the program design, which is out of our power to alter, when your downloading slots are full, you are prohibited to add any job, even it is an uploading job.

So, the solution to this is, stop all your downloads first, then start your uploads. After your uploads works properly (turns into green arrows), then start your download, and then you would not get that warning.

So, as a habit, "seed before you leech"!