lme4 is the name of the package. lmer is the actual model-fitting function from the package. So try:
# Load the package to make its functions available
library(lme4)
# Fit the model
my_model = lmer(y ~ x + (1|z), data=my_data)
To make it easier for us to provide additional help tailored to your problem, please provide a reproducible example. A picture of the RStudio window generally doesn't provide enough information and code and data can't be copied/pasted from it.