Reboot the computer.  If your program is on a network, reboot all computers.

If rebooting doesn't fix it, or if it fixes it but then happens again, it could be something else, such as the anti-virus program scanning it.  This link is for ProSeries for a similar problem, but likely applies to Lacerte as well.

https://proconnect.intuit.com/community/tax-return/help/server-busy-this-action-cannot-be-completed-...