What it means
The platform will not accept the volume you entered because it does not fit the symbol's rules for minimum size, maximum size, or the step between allowed sizes. It is closely related to the 'invalid volume' error and fixed the same way.
Common causes
A size below the symbol's minimum lot. A size above its maximum for one order. A size that is not a multiple of the volume step (for example 0.03 when the step is 0.05). A decimal typo. Or an EA calculating a size that rounds to something invalid.
Step-by-step fix
Open the symbol specification and note 'Volume min', 'Volume max', and 'Volume step'. Enter a size that meets the minimum, stays under the maximum, and is a clean multiple of the step. If your calculated size falls between valid steps, round it to the nearest valid one.
How to prevent it
When calculating position size from risk, round the result to a valid step before entering it. If you use an EA, make sure it normalises volume to the symbol's step. Keep the volume rules of your main symbols handy.
Broker's fault or yours?
A settings issue on your side, not a fault. It is a good reminder to check whether the minimum lot lets you risk as little as your balance really needs.