Full panorama isn't stitching

there are two showstoppers here:

• the missing parts of your panorama are more or less featureless areas (sea and sky). the pattern recognition skills of the PS stitcher are giving up here.
• photoshop can't "think" around the 0°/360° border horizontally and cannot close the image poles at +/-90°. it simply thinks ".

i've stitched a lot of  drone panos and recommend a professional stitcher like https://ptgui.com here. if you know the capturing pattern of your drone series, you can create a ptgui template for this and after a pre-positioning with suing the image meta data it will stitch these featureless areas way better than PS.

It looks like the pano should be three rows of seven, but it looks like you are missing the lower two frames on the left. Is that possible? 

As GoldingD said, shooting pano images vertically works much better. Also, 50% overlap will give you good results. For a multi-row pano, that would be 50% vertically and horizontally. 

You might want to try in PS.

1. The shoot, how many columns, how many rows?

2. The shooting order, it appears unstructured, what was the pattern? And what two photos failed? Can you annotate your screen shot to indicate what photos go together, say by column.

3. Appears to be multiple row multiple column. First assumption would be to go for spherical, although to also see if perspective helps with an odd shot. Only cylinder if spherical fails.

4. Did yo accomplish any develop module edits prior to merge attempt?  If so, try without.

P.S. you might want to shoot in portrait next time as to cut down on rows.