I am running a linear mixed models for a study about scRNAseq with the variable leiden( meaning different clusters) as a random variable. The thing is that I got what is called ‘boundary (singular) fit ‘: my random variable has a variance and Std.Dev. close to 0

my model is:
my_formula <- y ~ GENE_a+ GENE_b + n_genes + percent_mito + (1 | leiden)

myFit <- glmer.nb(formula(my_formula), data = allMat, control = glmerControl(optimizer="bobyqa", calc.derivs = FALSE))

and I got output like this: