GridSearchCV using Random Forest Reg Pipeline
The problem seems to be that your pipeline uses a fresh instance of RandomForestRegressor, so your param_grid is using nonexistent variables of the pipeline.  There are two choices (I tend to prefer the second):

Use rfr in the pipeline instead of a fresh RandomForestRegressor,  and change your parameter_grid accordingly (rfr__n_estimators).

Change param_grid to use the lowercased name randomforestregressor__n_estimators; see the docs on make_pipeline:

it ... does not permit naming the estimators. Instead, their names will be set to the lowercase of their types automatically.