Have you set serials for the SDRS and configured serials for the two services?
How to Install and Configure Piaware 5.0 SD card image - Quickstart Guide
Not sure which thread it is for non piaware sd-card instructions.
These should also make sense for setting serials: Raspbian Lite: ADS B receiver · wiedehopf/adsb-wiki Wiki · GitHub
If everything is good it could also be that the 978 SDR is getting insufficient voltage or something like that.
Pretty sure the claim interface error isn’t the real error … not sure though:
Jun 17 16:23:16 Raspberrypi3ADSB dump978-fa[6212]: SoapySDR: using maximum manual gain 49.6 dB
Jun 17 16:23:16 Raspberrypi3ADSB dump978-fa[6212]: SoapySDR: using stream setting buffsize=262144
Jun 17 16:23:21 Raspberrypi3ADSB dump978-fa[6212]: Message source reports error: TIMEOUT