There are several problems in your request:
- CRS is defined as EPSG:3857, yet your geometry is provided in EPSG:4326
- geometry should be provided in latitude/longitude order
- RESX and RESY should not appear in Statistical API, used RESOLUTION instead
- RESOLUTION should be provided in the same units as the CRS, e.g. in degrees in your case; there is an exception in case of Statistical API, that you can use RESOLUTION=10m if you want us to give a best try to convert these “10 meters into degrees at your latitude”
Applying all of this you get a request, that works:
http://services.sentinel-hub.com/ogc/fis/4f9e0919-3206-4690-MASKED?LAYER=VEGETATION-INDEX-NDVI&STYLE=INDEX&CRS=EPSG:4326&TIME=2018-11-15%2F2020-05-15&GEOMETRY=POLYGON((-27.98739282040455 -54.08952876561413, -27.98760544600711 -54.08336551369231, -27.98144772947314 -54.08459142114501, -27.97536276995694 -54.08738735387322, -27.97533191400677 -54.08843974280309, -27.97588577227436 -54.08908634398883, -27.97696274098633 -54.09016603092668, -27.97760384874638 -54.09139788028573, -27.98277768003278 -54.08986613010527, -27.98739282040455 -54.08952876561413))&MAXCC=5&RESOLUTION=10m
Note that I have masked part of your instance ID to prevent other readers of this forum to abuse it.