Because the Alexa service is down!
When the Alexa service is unavailable, you’ll get a reply from Alexa saying she’s “having trouble understanding right now – please try again later”.
Remember that your Echo speaker doesn’t transcribe your voice commands on the device itself. Instead, those commands are beamed up to Amazon’s servers, where they are processed. If the Amazon servers go down, your speaker basically turns into an overpriced doorstop.
Is there anything you can do when you get this message? Not really. If there’s a problem connecting to your Wi-Fi, Alexa will give you a different warning message, so don’t bother resetting the speaker or rebooting your router – it will make no difference.
Instead, you’ll simply have to sit tight and wait for Amazon to get its servers up and running again.
Now read this: What are the funniest things to say to Alexa?