At some point in the past Quicken started tracking cash in investment accounts. If you sell a security in an account, the cash will appear in the Cash Balance. Transfering some of that cash to (say) a savings account will decreast the Cash Balance.
If a security is purchased Quicken seems to want to take the money out of the Cash Balance in the account. And, I *think* it will get money from the attached account if needed. If there isn't enough money anywhere, I suspect it completes the purchase anyway and indicates you are short in that account. That means you must put in some money.
How do I clear out the indicated Cash Balance? How do I determine the basis for shares used to cover a short position?