In case anyone from TP-Link is watching, I think it should say... "There is NOT enough bandwidth. Please enter another one!" But, potato patato...close enough I guess. LOL.
I had the same problem with a TL-ER5120. When I initially set everything up I didn't have any problems. But over time as settings were tweaked/adjusted I got this error one day also. What fixed it for me was this...

Advanced
Traffic Control
Bandwidth Control

Then I lowered the up and down bandwidth options under "List of Rules". Your bandwidth on this page might be set too high with the "individual" setting. I think when I initially set mine up, some were "shared" and when I changed the setting it caused the "no bandwidth" error. Basically if your numbers are to high on the "List of Rules" it will give you this error. I would assume it's going off of the "guaranteed" numbers. Alternatively you could probably raise the bandwidth from the page your on but that would defeat the purpose of actually setting it up. Hope this helps.

"B"