I often get this for a number of reasons, one being I have a new mech with no tech attached, it seems like MMHQ wants to tell me to do that before I fight. Or, there's something weird with one of the mechs in the lance that's supposed to be fighting (it was headcapped in the last fight).
Or the above thing if it's a 'special mission'.
What I usually do is check my techs, check my mechs, check my pilots, undeploy them all, save it, reload MMHQ then you should be good to go. Has only happened in one in 50 or so fights for me, not a big problem.