SpeckleQGIS is normally installed within QGISs Plugin Manager, drawing from their plugin repository. It has been reported that that repository version will not install.
Having had the error reported for version 2.9.0 on both Mac and PC for different versions of QGIS, I thought it might be helpful to share the run through the alternative means of install.
The essential step is to ensure no copy of speckle-qgis exists within your active profile.
Repo link is here
