You cannot join Share Play because the network connection speed is poor, or the network connection is unstable.
To get the best experience from Share Play, both you and your friend will need a high-speed internet connection. The conditions surrounding the home network of each player may also impact internet connectivity. As a guide, a minimum connection speed of at least a 2Mbps is recommended. To check your upload speed, please check with your network service provider.
Hints for improving your network connection
• It is recommended that you use a LAN cable to connect your PS4 to the network.
• If you are using a Wi-Fi connection, place the PS4 and the router closer together, and make sure there are no obstructions between them.
• Other devices on your network might be using a large amount of bandwidth. Wait until they are finished.
• Check the connection status of the PS4 in [Settings] > [Network] > [Test Internet Connection].