Limitless exchanges are very hard to handle, as it becomes almost impossible to conduct market analysis or trade.
The minimum amount that can be bought or sold via trading platforms is called a lot. Changelly PRO sets a minimum (or lot) amount for each traded pair. For example, the BTC lot is 0.00001 BTC.
You can check the lot size in the "Buy" section on the "Trade" page. The minimal amount you can choose in the "Amount" field represents the lot size for the chosen asset. Just click the upward arrow in this field when the field is empty to see the minimum amount.
Here are the solutions if you cannot sell your crypto, as your amount is less than a lot amount:
Withdraw your assets
Keep the assets in your account for a more favorable future
Buy the missing amount