I'm new to selling on Etsy and found a message from someone that would like to purchase something but is having trouble with checkout. I tried to reply to him and I get an "unable to send" message. How can I fix this?
SCAM! Mark as spam.
You can't reply because etsy has already determined it's a scam account and removed it.
Scam and you're lucky by the sounds of you not being able to reply etsy has jumped in quick and saved you from being ripped off. Good luck for future sales.
@WittyWearsandWares: As the many threads about this in this forum indicate, It is a SCAM - you might want to read the excellent post by @bradgoodell in https://community.etsy.com/t5/Managing-Your-Shop/How-to-Spot-a-Scam-No-a-Buyer-Doesn-t-Need-Your-Ema... and https://community.etsy.com/t5/Technical-Issues/Why-do-they-ask-me-for-my-e-mail-in-order-to-make-a-p...
If the message is something like "User Name is Invalid" usually that message means that the account has been closed (either by the user or by Etsy).
There is also official information from Etsy at https://help.etsy.com/hc/en-us/articles/360000343128-How-to-Handle-Spam-and-Suspicious-Messages?segm...
If you have not already done so, you may want to read https://help.etsy.com/hc/en-us/articles/115015710308-What-to-Do-After-You-Sell-an-Item?segment=selli... for an overview how orders work for sellers.
Be sure to mark the message as SPAM so that it does not affect your stats.