how to fix FIxed effect model?

"The index on the time dimension must be either numeric or date-like" This mistake appears when i try to set up fixed effect within model and random effect model.

Ive got a panel data which contain year price cost and such numerical-only variables.

import pandas as pdfrom linearmodels.panel import PanelOLS

df = pd.DataFrame()

within_model = PanelOLS.from_formula('cost ~ RPM + price + load + EntityEffects', data=df)result = within_model.fit()

print(result)