In an IBKR Cash account, an investor must pay for the purchase of a security meaning, the trade must settle prior to selling that security.
A Broker can have rules that are more restrictive than required by regulators. IBKR's implementation of the free riding rule uses an end of day check to see if a position was closed prior to its trade settlement date. This IBKR end of day surveillance would still consider closing a position before settlement a Free Riding violation and would automatically put the account into "Cash Up Front" restriction for the next 90 days.
If an investor Sells to Close a position before the trade settles, then they can only place new orders with fully settled funds for the next 90 days. When this happens the values displayed in the Account Window such as the Total Cash Value of the account will include the settled and unsettled funds and the investor can place a new order when Available Funds (Equity with Loan Value - Initial Margin) is positive as that represents the fully settled cash after a closing trade. The real time check for fully settled cash is done during the Credit Check when entering a new order.
You can find more information on the Free Riding Rule in the IBKR Knowledge Base.
To request an upgrade to a Margin account and be allowed to trade using unsettled funds please review: Can I upgrade to a margin account while a free riding restriction is in effect?