Aha - I knew there was a reason why I stopped doing this.
If you’re a Gmail user, Google actually won’t let you attach .apk files in case they contain malicious code.
What I used to do was move the .apk into a Google Drive folder and the you can share a link via email, or you can use the Google Drive app to open it on your phone.
To be honest, what I’ve found to be quickest now is just to log into Thunkable X on my phone, open the project I want, click download and then the .apk downloads directly to my device. Once it’s downloaded make sure you close the tab with Thunkable open on your phone.
Hope that helps.