To find devices, Echo needs to meet the following requirements.
1. Make sure that the app account that binds with Echo and the account that the device is on is the same.
2. Make sure that the device is online. Devices can't be found if they are offline.
If the device still can't be found, try using the web version of Alexa to search and control the speakers since the Alexa app can be unstable in some regions.