colnames(df)
COL_A, COL_B, COL_D

output <- lmer(COL_A ~ 1 + COL_B + 1|COL_D, df, REML = FALSE, verbose = TRUE)

var_a<-COL_A
var_b<-COL_B
var_c<-COL_D

output<-lmer(var_a ~ 1 + var_b + 1|var_c, df, REML = FALSE, verbose = TRUE)

Error in model.frame.default(data = df, drop.unused.levels = TRUE,  :
  variable lengths differ (found for 'var_b')

form_lmer<-"COL_A ~ 1 + COL_B + 1|COL_D"

output<-lmer(form_lmer, df, REML = FALSE, verbose = TRUE)