The issue is you can’t build more if you have already built 6. So you need to sell at least one before that.Doing the same although im going for 6 as their drop rate is quite good. Replace the burial chambers. Then build and sell the others.
And an advise from me - don’t sell your burial vaults. You should never sell a single pax building - it’ll come time in the game you I’ll need more than you have