Yeah it sounds as if it is from another beacon or an alt and there are plots above/below the plot you’re trying to claim.
If it is part of another beacon you would have to plot down(or up) from the existing plot in that column. Similarly if it’s an alt that has a plot higher or lower you would need to use that alt and plot down or up from that plot.
Hard to explain, but check that no other plots are in the ‘column’ of plots you’re trying to plot.
…Plot the word is losing meaning
Edit: plotting any altitude will reserve all plots above and below from mantle to sky, so only plots from the same character and same beacon can be plotted in one column.
So if you stand in the plot you want to claim, remove all plots above and below and then put down a new beacon it will let you claim. However when you reclaim you will then lose that entire column if you plot it all…