You have to disable screen security in order to take screenshots. Screen security, when enabled, shows a blue screen with the Signal logo if you are looking through your list of running apps.
Follow the steps below to disable screen security and enable screenshots.
- Open Signal.
- Tap on the Menuor .
- Choose Settings.
- Choose Privacy.
- Tap the "Screen security" option. If you see the slider to the right with a blue color, then you now have enabled screenshots.