[R] error message for function: lmer (from lme4 package)

[Fix Ace]

Dear R Community,
My data have 3 conditions and each condition has 6 replicates. I am trying to fit my data for a linear mixed model using the lmer function from lme4 package to find the random effects of the replicates; however, I got the error message. Here are the example codes:
>example.3=data.frame(levels=as.numeric(XXX[,c(4)]),replicate=rep(c("0","1","2","3","4","5"),3),conditions=c(rep("11",6),rep("12",6),rep("13",6)))> example.3    levels replicate conditions1  43.1111         0         112  42.0942         1         113  57.8131         2         114  57.1726         3         115  77.8678         4         116  44.7578         5         117  69.5078         0         128  52.0581         1         129  40.0602         2         1210 45.5487         3         1211 43.6201         4         1212 60.4939         5         1213 64.1932         0         1314 53.4055         1         1315 59.6701         2         1316 52.6922         3         1317 53.8712         4         1318 60.2770         5         13> m.example.3=lmer(as.numeric(levels)~conditions+(conditions|replicate),data=example.3)Error: number of observations (=18) <= number of random effects (=18) for term (conditions | replicate); the random-effects parameters and the residual variance (or scale parameter) are probably unidentifiable> 
Could anyone help me figure out how to fix the issue? 
Thank you very much for any inputs!
Ace

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