>>1561377
The color palette on your map must be higher than 64 colors, check this https://worms2d.info/Colour_map#Colour_limitation

To fix it, depending on the software you are using to make the map you can limit the palette to 64 or compress it after you're finished. You can also run it through one of those online image compressors and tell it to reduce it to 64 colors. Give it a try and see if it works.