> what can I do if I did that regression using 3dDentred? Can I get the errts in an isolated step?

3dDetrend is all you did at the individual subject level? If so, you may use the output from 3dDetrend as an approximation.

> Are these parameters the average across all volumes/subbriks?

Yes.

> Should I put the first three in acf option when using 3dClustsim then?

Yes.

> if I get every subject acf parameters, should I put the mean of all these inter-subject acf parameters with the tstat map as
> an input (using 3dClustsim) ?

You can use the average ACF parameters across subjects as input for 3dClustSim with a brain mask that has the same resolution as your group analysis results.

> if I am using correlation map for the group level analysis, Is it possible to use the option -resid in 3dLME to get the residuals
> for each subject's correlation map and then calculate the acf parameters from these residuals (coming from subject correlation
> maps)? or is getting the errts file mandatory?

Either is fine as an approximation for the ACF parameters.

> from which type of data should extract the errts? from subject EPI or subject statistical maps?

Probably preferable to use the residual time series.

Gang