I know very little about Thunderbird and I get "that account already in use..." Support forums indicate something in about:config would indicate problem.
I have copied the about:config portion suggested in forums but do not know how to tell which is bad account, or if none are bad. Would appreciate some help. server setting: that account exists, choose another name. These are the results of about:config. How do I tell which one is bad? I suspect the last because of the "m6aitly2.default". mail.server.server2.userName;nobody mail.server.server2.type;none mail.server.mail.server.server2.storeContractID;@mozilla.org/msgstore/berkeleystore;1 mail.server.server2.spamActionTargetAccount;mailbox://nobody@Local%20Folders mail.server.server2.name;Local Folders mail.server.server2.hostname;Local Folders mail.server.server2.directory-rel;[ProfD]Mail/Local Folders mail.server.server2.directory;C:\Users\Barbara.Young\AppData\Roaming\Thunderbird\Profiles\m6aitly2.default\Mail\Local Folders
ప్రత్యుత్తరాలన్నీ (1)
I think that rather than you guessing at what's wrong, it would be better to share with us the exact words of the error message.
One interpretation could be that you're using a server that accepts only one connection at a time, and so Thunderbird's attempt to connect is being refused because the server is already or still connected in some other way. I recall an account where if you connected using webmail, it could take up to 10 minutes after logging off for the account to become available to an email client.
OTOH your message might mean that Thunderbird is refusing to allow you to add a further account on the same server.
Or that your profile is busy and therefore locked.
These all have different solutions, and none of them AFAIK involves the config editor. So a link to the help that suggested this to you might also be useful.