請將遊標置於正文欄位所需的插入點

What happened

You tried to insert a signature into a mail message, but the focus is on one of the address fields, and you can't add the signature there.

What you can do

Click within the rich-text area of the message where you want the signature to appear (usually at the bottom), or use the arrow keys to move the cursor down to the rich-text area.