There could be various reasons to explain why your clients are unable to reply to Facebook or Google reviews — these include:
1. Facebook and / or Google profiles are not connected to the report
To resolve this for Google, for example, simply click the green 'Connect GMB Account' button (indicated below).
2. Client Access to Reputation Manager is not enabled
You will need to ensure that the 'Enable client access for Reputation Manager' option is switched on (see the red box below).
3. Access tokens have expired
This means that Facebook and / or Google profiles need to be reconnected by clicking on the blue 'Please click here to give us access' link that you can see below.