Possible cause

WARNING: This page is about a different car, the 2017 Mazda 3. However, it is still accessible from the selected car via links, so may be relevant.

Connector terminal disconnection, poor contact, damage, deformation, corrosion
Electric parking brake control module power supply voltage or body ground malfunction
Open circuit in wiring harness between electric parking brake control module and connector C-40
Connector C-40 malfunction
Electric parking brake control module malfunction