Why fitlme does not calculate random effect term?

I tried to fit a simple lme model using fitlme function: lme = fitlme(tblData, 'D ~ 1 + (1|U)'). Why it does not calculate the random effect term for my data: Esitmate = 0 and Lower (Upper) = NaN? The data file is attached.

Can any one give me a clue?

Linear mixed-effects model fit by ML

Model information:

Number of observations 30

Fixed effects coefficients 1

Random effects coefficients 3

Covariance parameters 2

Formula:

D ~ 1 + (1 | U)

Model fit statistics:

AIC BIC LogLikelihood Deviance

19.377 23.58 -6.6883 13.377

Fixed effects coefficients (95% CIs):

Name Estimate SE tStat DF pValue Lower Upper

{'(Intercept)'} 2.826 0.055211 51.185 29 5.4902e-30 2.713 2.9389

Random effects covariance parameters (95% CIs):

Group: U (3 Levels)

Name1 Name2 Type Estimate Lower Upper

{'(Intercept)'} {'(Intercept)'} {'std'} 0 NaN NaN

Group: Error

Name Estimate Lower Upper

{'Res Std'} 0.3024 0.2348 0.38947

[wangm]

looks like few people use linear mixed effects model?