Hello,
Intelligent capture is one feature in CatC where we require direct reachability between the access points and DNA Center.
GRPC link is not ready could possibly be the result of the following:
1. AP unable to reach DNA Center over port 32626
2. DNA Certificate in invalid on WLC
If the AP uplink device is a catalyst 9k, you can run an embedded packet capture on the AP uplink interface and see whether or not you see successful communicate between AP and CatC on port 32626. This is also helpful to validate the SSL handshake that occurs between AP and CatC.
HTH!