Augment is not defined for a twfeCovs fit (documented omission)
Source:R/broom_methods.R
augment.twfeCovs.Rdaugment() is intentionally not provided for twfeCovs() objects (#58):
the fit's coefficient vector lives in a reduced cohort-level basis that the
shared fitted-value path (X %*% beta_hat) does not match, so a meaningful
.fitted / .resid cannot be reconstructed. Use tidy.twfeCovs(),
glance.twfeCovs(), or summary() instead. Calling this method always
raises an error.