What does maximum deviation mean?
Maximum deviation means that you determine a range of prices, in pips, where your order can execute.
Note: MT4 executes ALL orders as FILL or KILL. Meaning, the entire order will have to be filled at the same price (or within the range of prices if you are using Maximum Deviation) or the order will not be filled.
If you have maximum deviation set very low (1 or 2 pips for example) and are trying to execute a large trade, if enough liquidity is not available in the price range you requested, the order may not execute. To help avoid this, you may consider increasing the maximum deviation or deselecting it all together.