Using PCA to cluster multidimensional data (RFM variables)
Judging from the plot, there are no clusters.

K-means requires continuous variables to work well. The data you have has discrete steps (which causes the grid pattern in your plot).

There is no benefit of using PCA here. Use it only for visualization. The scale -3:3 that you don't understand is from PCA. So you probably have not understood PCA enough either.