Then comes Der von dir eingegebene Mitgliedsname gehört keinem gültigen Etsy-Nutzer. The user is not from the Etsy system!
What should I do now?
You must send your [ᴇ-ᴍᴀɪʟ] in this chat. You will receive an [ᴇ-ᴍᴀɪʟ] from 𝗘𝘁𝘀𝘆◎
You are being scammed!
This is a SCAM. When it says "not a valid user", that means that etsy already identified them as a scammer, so deleted their account.
If they said you got a sale, you didn't. If they said your payment account is paused, it isn't.
Mark the message as spam and ignore it. DO NOT ENGAGE.
@ThoughtisartShop: 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...
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...
Real messages from Etsy are now going into their own section on your messages page https://community.etsy.com/t5/Announcements/New-From-Etsy-section-in-Messages/m-p/143722497#M1899
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.