library(brms)
library(projpred)

J <- 1	
N_per_site <- 84	
site_id <- rep(1:J, rep(N_per_site, J)) 
index <- rep(1:N_per_site, J)  
N <- length(index)
time <- index	
cont1 <- rep(0,50)	
treat1 <- rep(1,34)	
treatment <- c(cont1,treat1)
	
alpha <- 2.7	
beta1 <- 0	
beta2 <- 0.01	
beta3 <- -.01
lambda <- exp(alpha + beta1*time + beta2*treatment + beta3*treatment*time)
outcome <- rpois(N, lambda)

data <- cbind(time,treatment,outcome,site_id)
data <- data.frame(data)
data$treatment<-factor(data$treatment)
data$site_id<-factor(data$site_id)
data$month <- rep(1:12, times=7)

plot(outcome ~ time, data=data)

m.test <- brm(outcome ~ time*treatment + s(month), data=data, family=poisson, cores=4)

vs <- varsel(m.test)

#Error: Projpred only supports standard multilevel terms and offsets.