This message appears if the character selected is not in the font, or if there is no character in the Letter text box. Such characters include square brackets [] and braces {}. Select a valid character from the popup menu.
This message appears if the size entered in the Size box in the Insert Lettter dialog is below 3mm or above 200mm, or is not a number. The Size setting refers to the height in millimeters of an uppercase 'A' in the selected font. If you wish to increase letter above this you could drag one of the corner boxes to resize of rescale it.