I am not able to fit SED data with zero-value uncertainties - why?
When a SED is fit using a chi-squared statistic in Iris, any data points with associated zero-value errors are excluded from the fit. If the fit is done with least-squares or maximum likelihood statistics, then the points with uncertainties set to zero are included - because the uncertainties are not used anyway by these statistics.
This design choice is intended as a safeguard against yielding potentially misleading fit results in the analysis of your SED data. Iris interprets a zero-value error not to mean that the uncertainty on the associated data value is actually zero, but that a measure of the uncertainty is not available for that particular photometric point.