If you haven't installed it... try downloading it to your device from the MEGA app, then going into whatever your phone has as a file manager (usually called "my files", "files", or "file manager") and navigating to wherever you set it to download the app, then tapping on the APK to install it. You may need to set extra permissions to install it. Once it's installed, you're good to go.
If you've already installed it, I have absolutely no clue how to help you there. You may need to delete the application from your device, then backup your device, and factory reset it, and try again. Make sure everything you want to keep (like other apps' saves) is backed up first, I can't stress this enough. Heck, it may just be that your phone needs a restart. Or that your phone can't run it due to minimum requirements.