Hi, and welcome!
There are no stupid questions here. We're a community of learners from beginners through intermediate and advanced, with occasional visits from on high--some of the top luminaries in the R universe.
One of the things that allows us to be helpful is reproducible examples, called a reprex . In the case of the code snippet, dataset is not in the namespace by default. Representative data in the same form (even from a built-in data set) makes for more and better answers.
In this case, though, running the example from help(package = Rfit, rfit) illustrates the problem:
library(Rfit)
data(baseball)
data(wscores)
fit<-rfit(weight~height,data=baseball)
summary(fit)
#> Call:
#> rfit.default(formula = weight ~ height, data = baseball)
#>
#> Coefficients:
#> Estimate Std. Error t.value p.value
#> (Intercept) -228.57143 56.14659 -4.0710 0.000146 ***
#> height 5.71429 0.76018 7.5171 4.373e-10 ***
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Multiple R-squared (Robust): 0.4418605
#> Reduction in Dispersion Test: 45.125 p-value: 0
Created on 2020-01-03 by the reprex package (v0.3.0)
Then, take a look at the rfit function signature
Default S3 method:
rfit(formula, data, subset, yhat0 = NULL,
scores = Rfit::wscores, symmetric = FALSE, TAU = "F0", ...)
subset is optional, but it needs both formula and data. The error thrown by the code snippet reflects the missing data argument, which would tell rfit where to find xfull.