Option "-polort 1" means to use the polynomial t^0 and t^1 as regressors -- that is, to detrend a constant (mean) and a linear slope. That is why you have two regressors. (I'm a little surprised CompCor doesn't do this itself, actually.)

The "18 vectors are constant" warning says that in your input time series dataset, of the 622,592 voxels, 18 of them were constant in time = 0.00289% . Almost certainly, these voxels are outside the brain. The point of this warning (it is not an error, otherwise it would say ERROR instead of WARNING) is for cases when a LOT of voxels are constant -- which is peculiar, unless some part of the data has been deliberately zero-ed out earlier.

A voxel time series that is constant in time will, after removal of the linear trend, be all zero. It is quite likely that these voxels were all zero to start with, but the program doesn't check for that as a special sub-case of constant.