Some apps will require you to pair your trainer as the source of your cadence data in a separate step from pairing it as a power source and controllable trainer. Look for the option to pair a cadence sensor in the app's "devices" or "connectivity" menu or equivalent, search for devices and select the Fliiiight trainer. Check the app's support resources for more assistance.
If the cadence value populated previously:
Follow the general troubleshooting steps for 0 readings.
Check the app's support resources for more assistance.
Some ERG mode apps choose not to display speed by default, due to its lack of relevance in indoor training.
Apps designed to track outdoor activities may use GPS-based speed. In this case, since you are stationary they should report the speed as zero. Extremely low GPS speeds may also be reported due to minor errors from the GPS. The 4iiii app only displays speed based on GPS.
If the speed value populated previously, or if there is also an issue with your power readings, follow the general troubleshooting steps for 0 readings or check the app's support resources for more assistance.