I had no H1 heading before so followed the following thread, inserted the code accordingly and it worked out for the H1.
However, I checked it out again with the tool as mentioned by Kharis,
This time I got this error message, the H2 and / or the H3 are missing.
Prior to inserting the code for H1, I had H2 and H3 but no H1 at that time. Do you have any idea to fix this?