Help with firing smoke

[PawelM]

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I found some mortars (I think 60mm) are not capable of firing smoke. You can check if unit is capable of firing smoke by clicking on it. If in the lower left corner you can see white/grey-ish smoke ball then the unit can lay smoke (I hope it is visible in the image attached). I found that if the mortars cannot lay smoke the error message is somewhere along the lines you quoted. So it seems to me this particular mortat unit was not capable of firing smoke shells.

...And of course as Mr. KK said you need to have smoke available and number of smoke shells varies from scenario to scenario and often is 0.