So yesterday we tried to pitch some tents in our base but it didnt work. We thought it might be because of the netting?! we removed that, still we got the error message saying the ground needs to be free and flat. There was definitely enough room for the tent, and the ground was as flat as it comes. Does anyone know what might cause the problem?`Base consists of netting and containers. Outside the base it worked just fine.

It's fixed in the new dayz patch, they updated code after 1.7.6.1 to make it so people can't place tents inside other objects for performance reasons. Unfortunately they didn't get everything right and there are still bugs with it currently. It's probably your base itself that's stopping you being able to build it rather than the ground.

Until 1.7.7.2 is released I would either take some of your base down before you try and build the tents or build the tents somewhere else first then build the base around/ontop of the tents.