Sharing songs and sets using the Share Menu in OnSong uses the built-in Mail app to compose and send attachments. If you are using a third-party email app, the built-in email composition tool cannot function. You will need to configure an account in iOS/iPadOS in order to use this feature.
To do this, go into your device's Settings app from the home screen. This is where you normally connect to different WiFi networks. Next, choose the Passwords & Accounts option and choose Add Account. You will need to add an account there for your email address. Please contact your email service provider for details on how to use your email account on your iPad or iPhone. Once this is done you'll be able to email from OnSong.
Unfortunately having a third-party email app installed will not work. This is because it is not possible to send attachments from one app to another.