How can I fix a tear in my nylon coat?

There is rip stop nylon with heat sensitive glue that is sold to do such repairs. your iron the mending tape over the rip.That repair might work for years and years, or it might come off during the next rain.

https://www.amazon.com/KENYON-K-Tape-Repair-Tape-Ripstop/dp/B00PUCH70E

I had some down comforters that were torn. I used this. It worked great! https://www.mec.ca/en/product/5008-815/Tenacious-Tape-Repair-Tape

Did mine hand stitch close mend but hardly see good luck !

I found a youtube video that shows how to repair a puffer coat. I think the material would be treated in a similar manner. I hope this helps...

https://youtu.be/BFU8X-6G7vc

There are peel & stick patches for this found in sewing centers. There are iron-on patches as well but please keep in mind that extreme care must be used when putting any iron on this kind of material. High heat can sometimes melt it. Also REMEMBER, NEVER put it in a clothes dryer as heat from the drier can cause glue to turn loose.

Stitching the tear closed would likely be the most durable but doing so without it showing would also be a concern. If you could come up with a stitching pattern that would make the repair " appear" as though it were intentional rather than a repair, that would be a great choice.

In considering stitching the tear, might it be possible to also stitch a large designer style patch on it to cover it & yet also make the coat " appear" as tho it just came that way? Even an additional pocket or 2.

Bling is quite a thing in today's fashion world. Maybe you could actually stitch the tear then put a pattern of bling design on the coat to cover or camouflage it. A leather or contrasting fabric might also work.

You didn't say where the tear & spot actually are. Explain & perhaps I could offer another solution as well.

Hope this might help.