There is a known issue where chords might unintentionally look like chords blocks in the preview mode of Yousician right after uploading. These chord blocks should go away after you save the song.
If you would like to see chords as chord blocks, you need to define them in the Guitar Pro file. To do this, select the chord you’d like to see as a chord block in the game, and press ‘A’ to bring the chord name dialog.
Select the chord name, and click ‘OK’. If the chords have chord names on top of the staff (as in the image below) in Guitar Pro, they will be displayed as chord blocks in the game.