Error in class.plot

[linxianxii]

安装好R包时，基于 forestData 数据建立模型会出现Error in nlme.formula(Bio ~ a * (1 - exp(-b * (S/1000)^c * AGE))^d, data = data, :
Singularity in backsolve at level 0, block 1
Error in class.plot(data, model = "Richards", interval = 5, number = 5, :
could not find function "class.plot"