Really need a bit more info here, why are you subsetting the first 15 columns? Did you define n before id = rep(1:n,rep(nBlocks,n))? But back to the error code, it usually indicates multicollinearity, check for any NAs and run something like

sapply(z, function(x) sum(is.na(x)))

cor(z[sapply(z, is.numeric)])

library("ggstats")

library("apollo")

library("readxl")

library("mlogit")

library("dfidx")

library("haven")

library("ggplot2")

library("logitr")

library("parallel")

library("dplyr")

library("fastDummies")

library("nnet")

library

("guideR")

#########################################################################################################################

# Step 2: Load data, detect cores, socioeconomic characteristics:

## Step 2.1: Load data:

database_orig