How do I solve Secret Note #16?

Solution 1:

Double-check whether you are digging at the right spot.

Since version 1.3, you have to collect secret note 16 before you can dig the treasure, but you should be able to dig at the spot without the note. Make sure that you dig at the right spot:

Dig one space to the right of the large boulder north of the Railroad tracks in the top left corner. See https://stardewcommunitywiki.com/File:SecretNote16.png