To estimate a panel data using the within model,we use the plm package

I need counts of all levels used in a glm, including the reference levels. Here is code I am using with an example dataset:

I am performing an OLS regression with time and industry fixed effects and I want to compare a potential selection problem with the Heckman approach in R.

My objective is to run a linear regression model with no intercepts, but my data is grouped in the target variable.