I can't respond to customers message I keep getting an error
More info needed?
What kind of error?
Since you're a new shop, it could be a scammer wanting you to re-verify your shop so you won't be suspended and/or asking for your email so they can send you off site to enter in your bank/cc info to STEAL your money or wanting to send you a QR code.
And if this, Etsy deleted that account before you could get scammed so you'd get an error that the user is invalid or something similar.
https://community.etsy.com/t5/Announcements/Protect-Your-Account-From-Scammers/m-p/143158945#M1882
https://help.etsy.com/hc/en-us/articles/360000343128-How-to-Handle-Spam-and-Suspicious-Messages
One other thing it could be is that you're sending out unsolicited messages to people that have favorited your items (which is against the rules and will get your messages disabled for a period of time 1-2 weeks.
We need to know what error message you are getting when you try to send your reply.