Mouse clicks and key strokes do not work in the antivirus software window opened on the remote PC during a remote session in the Full Control mode.
Many popular antivirus software packages have a self-protection system/feature in effect. This feature prevents the software settings to be manipulated remotely. Note that different antivirus programs may call this feature differently.
You can disable self-protection by directly accessing the antivirus software settings on the remote PC.
For example, in Kaspersky Internet Security the feature is called "Self-Defense" and is available under Settings ➝ Advanced settings. If you are using a different antivirus software package, refer to its documentation for a feature called "self-protection", "self-defense”, etc.