Did you bid on the plot? If so, did you win the plot and are unable to relocate?

If you didn't bid/win the plot, then you can't relocate - it's not free reign.

If you did win and you are getting an error, then your storeroom status is most likely preventing you from moving.

Normally, when a plot is unavailable at the time, it means it was vacated during the entry period for the lottery. You would have to wait until the next entry period when it would become available to bid on it.