lme4 in R error: boundary (singular) fit: see ?isSingular

I am trying to run lme4 package in R. I have 10 Lines in total with four plants for each line in each of the two replications. But some of the plants died and there are some missing values. Weight is the response variable. Here are some lines from the data:

Line    Rep Weight  PLANT
Line 1  1   NA  1
Line 1  1   NA  2
Line 1  1   NA  3
Line 1  1   NA  4
Line 2  1   26  1
Line 2  1   26  2
Line 2  1   26  3
Line 2  1   27  4
Line 1  2   26  1
Line 1  2   28  2
Line 1  2   26  3
Line 1  2   25  4
Line 2  2   24  1
Line 2  2   26  2
Line 2  2   25  3
Line 2  2   NA  4

I want to run linear mixed model using lme4 package so I tried running:

lme4 <- lmer(Weight ~ 1 + (1|Rep:Plant), data=Data)

But I got an error: boundary (singular) fit: see ?isSingular

I am using it for the first time and I am not sure about the error. I will appreciate any help!

Thank you!