I'll help you resolve the issue of being unable to capture signatures for invoices using an Android phone.
To optimize your device's performance, start by clearing the app data.
If you're using an Android device. Here's how:
Go to Settings and select Apps.
Choose QuickBooks Online and click on Storage.
Select Clear Data.
For iOS:
Go to Menu ☰ and tap Help & Feedback.
Tap Refresh Data and then Refresh to confirm.
Once done, restart your phone and reopen the QBO app. You should now be able to capture signatures in invoices. Read this article about the detailed steps of getting a signature using the QuickBooks mobile app: Capture signatures using the QuickBooks Online app.
Let me share an idea about the customer's signature on invoice. The reason why you can't find the feature custom to sign is that the option is not available on QuickBooks Online Apps using Android.
I suggest you check this article, to easily compare feature availability on supported devices: