@Joseph Milton depending on how you deployed, it appears to me the packages from the requirements.txt didn't get installed. To ensure this is being done, make sure your app service has SCM_DO_BUILD_DURING_DEPLOYMENT
set to True
under your app service Application Settings blade. Normally, this is done automatically if you're deploying using Visual Studio/Code or Azure CLI az webapp up
command. If you're doing GitHub Actions, make sure you yaml file is configured properly.
I created a basic Flask app in Visual Studio and added gTTS package and didn't have any issues with the web app loading
from gtts import gTTS
from flask import Flask
app = Flask(__name__)
tts = gTTS(text="Hello", lang='en')
tts.save("hello.mp3")