You have reached the maximum number of Caravan Trade; it reached the highest amount of resources that could be donated.

If you have exceeded your capacity and you can figure the capacity by entering the military market and choose the alliance you want to support, then you will see a page (support alliance); at the bottom, you will find capacity that you shouldn't exceed it and the added tax for your support. You can increase the capacity and reduce tax through upgrading the military market building.

Note:
You cannot donate protected resources.