It means that the plot that you are looking at was recently abandoned or auto-demoed and won't be available until the next lottery cycle.
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The lottery consists of a 5 day bidding period, when people have opportunity to bid on 1 plot, and a 4 day results period, when the winners of the bidding period may finalize their purchases. You were probably trying to bid during the results period.
A new bidding period has actually just begun as of several hours ago. All of the houses that previously said "unavailable for purchase" should now be up for grabs. Any houses that are demolished during this bidding period will not be available for bids until the next bidding period.
Make sure that you are bidding on a plot available to private buyers, and if it is a free for all plot, make sure you place your bid as a private buyer. You don't want to end up like the OP, who I strongly suspect placed a bid in an FC ward for the purpose of relocating their private plot.