Am I understanding correctly that your pool is defined over the 11 years?
If so then the reason is exactly what the error suggests. The default Swamy-Arora method requires the estimation of the between estimator which has as many observations as you have pool identifiers. It sounds as though you have more regressors than the number of pool identifiers.
In contrast, the other two estimators do not require this auxiliary regression. As we note in the manua