Well yes I have been around so I should take it in stride. How ever that is a big price to pay to continue the quest line. I did not post the title well and I rarely use the forum anyway. I guess I am just getting board with the game and if I could find a suitable replacement I would. Chalk it up to an old man's grumpiness.
Would be nice if you can post the title so we know exactly what quest it is but it's okay as I think I found the quest. I believe it's titled
Peace Movement and yes I see it's "Sell 4 military buildings from your age". Looking at your city you have the room to build a Heavy Weapons Center, sell it and repeat 3 more times. And just in case you might not be aware you don't have to wait until it's completed building. That will cost 732,000 Coins and 776,000 Supplies to complete that portion of the quest.
I mentioned your start date not to be judgemental but because I figure anyone playing that long would clearly have enough coins and supplies that it wouldn't even be a blip on the radar to do the quest but I see you started on U world I guess back in June/July of 2017 and that is not your main city but even still I would think just over a year now in one city you should easily be able to cover the costs of coins and supplies.
You say it's not about the coins and supplies though and about units and diamonds. As others have mentioned this doesn't make any sense as you don't need to do anything with units or diamonds regarding building and selling the military buildings, just plant the new military building and sell it off right away, the only costs you'll incur are coins and supplies.
Just another tip but it's important to play close attention to the wording regarding quests and be creative with them. There are many examples but here is one, if a quest requires you to delete some units you don't have to delete your units you have but you can make Spearfighters real quick and delete those units instead of your current age units. Of course if it says "from your age" then it's a different story and you'll need to delete ME units as that is the age you're in. Since you have a level 9 Traz that gives you 7 units a day which is only a day's and a half worth or so of units to delete.