I'm working with a random effects model for a panel dataset for a number of retailers and their daily sales. Since I am trying to infer the effect of an interaction between two independent variables for the different retailers, running a random slope for this interaction became too complex and so I switched to the approach of modelling each retailer separately.
Since the data for each individual retailer won't have any of the random effects that I was modelling (they were all based on 1|Retailer), I was trying to run a plm(model="within") fixed effects model for each one. However, since each of these subsets is a time series dataset rather than a panel one, the formula keeps telling me that the returned model is empty. Is my approach wrong? Should I aim to approximate each one simply using OLS with a factor() control variable for months and years?