How to fix "boundary (singular) fit: see help('isSingular')" in a LMM

I'm doing a LMM by using lmre4.
I added 2 fixed factor (i.e C/N and Substrate), and a random factor (i.e. ID)
This is my model:
"d.df$ID <- factor(d.df$ID)

y <- lmer(Ec.En ~ C.N + Matrix+ (1|ID),data = d.df)"
I had this message:
boundary (singular) fit: see help('isSingular')
Can someone help me?
Thank you