This message may pop up while looking at the mission book's weapons pages, if one of the pages refers to the Strella AA missile. The message may also appears during a game, as well.
In some editions of OFP (possibly the European editions), this error message has appeared since OFP's first version and through its latest version as well. The error is caused by OFP having problems finding the picture of the Strella's missile that should normally appear under the weapons tab in the mission book. Other than the message, though, the Strella works just fine during gameplay.