It might be helpful if you post your whole code. But as far as i remember the modelFit(fit1) function provides some statistical quality assessment for a drc object. So you need to first fit a model to you data with LL.4 as an argument; thus creating a fit 1 object. This should look something like this: fit1

Actually modelFit allow checking the fit of dose-response model only for coninuous and binomial data. For continuous data the clasical lack-of-fit test is applied. For quantal data the crude goodness-of-fit test is used. If modelFit return "No test available" it could mean that you try to check a model build with poisson data. For this type of model there is no test implemented. You can try your own (e.g. https://www2.stat.duke.edu/courses/Fall17/sta521/knitr/Lec-9-Poisson-Checks/predictive-checking.pdf)