Error Code in WGCNA: Dealing with large data sets: block-wise network construction andmodule detection
The problem is that blockwiseModules split your data into (probably 3) blocks, because the default maxBlockSize is 5000 and smaller than the number of genes in your data. If you have enough RAM (16GB is recommended for 12k genes, but 8GB just might be enough), rerun blockswiseModules again with maxBlockSize raised to above the number of your genes. Since you were able to run the function with maxBlockSize=8000, hoepfully you'll be able to do the single block calculation.