There is a limit to how many keystrokes a computer can retain ahead of what it is doing in real time. When the number of keystrokes entered reaches that limit, no further keystrokes can be entered and the warning tones indicate this fact to the user.
Try reestarting the computer to clear the error.
If the error persists after restarting, then probably the spring loaded mechanism under the key in question has become stuck in the depressed position. If so, try to gently lever the key up with the flat blade of a table knife, avoiding, if possible, unclipping the key itself.
If this is not possible, you might have to deliberately unclip the key to see if the mechanism under it is damaged. If it is not or it can be unstuck, then carefully re-clip the key.
If none of the above are possible, it may be necessary to replace the keyboard.
I hope this helps.
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