What it means
'Invalid volume' means the lot size you typed is not allowed for that symbol. Every instrument has a smallest lot, a largest lot, and a step size in between. Your number has to fit those limits exactly.
Common causes
The volume is below the minimum (for example 0.001 when the minimum is 0.01). It is above the maximum allowed in a single order. Or it does not match the step — if the step is 0.01, then 0.015 is invalid because it is not a whole multiple of the step.
Step-by-step fix
Open the symbol's 'Specification' and read 'Volume min', 'Volume max', and 'Volume step'. Enter a size that is at least the minimum, no more than the maximum, and a clean multiple of the step.
If you need a bigger position than the maximum, split it into two orders. If you need a smaller position than the minimum, that symbol simply cannot be traded that small on this account.
How to prevent it
Round your calculated position size to a valid step before entering it. Keep a note of the min, max, and step for the symbols you trade most, as they differ between instruments and brokers.
Broker's fault or yours?
This is a settings issue on your side, not a fault. It is a good prompt to check whether the minimum lot on your account matches how small you want to risk — some accounts do not allow the tiny sizes a small balance really needs.