It might be about minumum trade quantity. Maybe you need to buy at least x thousand of rdd. (don't know, maybe 3k or 4k?)

this is from bittrex:
https://support.bittrex.com/hc/en-us/articles/115003004171-What-are-my-trade-limits-
Minimum Trade Size: The minimum trade value for orders is 100,000 Satoshis (.00100000). There is also a minimum trade quantity. For example, DOGE trades at 22 satoshis. The minimum trade quantity is 4500 DOGE and the minimum trade value is 100,000 Satoshis. The smallest order in the book is 100000 DOGE at 1 satoshi each. The largest order in the book is 4500 DOGE at 2 btc each.