I took this photo at night with an iso of 100, aperture of 16, shutter speed of 30 seconds in raw. I want to send it to print but when exported to jpeg its only around 8.4 megabytes and the print site says that it may be too low quality. I did take it in raw, edited in lightroom and now exporting. The quality is set at 100 and the DPI at 300. Am I exporting it wrong? Was there a better way to take the photo?
The print site is most likely just using a simple algorithm to check the file for compatibility to print well. That being said, I'm unsure why they would be making this determination off of file size alone. As has been commented, jpg compression can work wonders in shrinking a file. Given that your image is mostly dark and 1/5(ish) of it is dark sky - your file size is probably the result of the compression. There's nothing inherently wrong with this.
The print site could actually be looking at the amount of file data instead to make it's determination, asking itself, "is there enough data here to print?" I imagine it could be looking at your intended print size, the photo's length and width in pixels, and the resolution - and then determining to alert you to make sure the file is large enough.
You would need to post the actual file you're attempting to print, or at least it's dimensions and resolution, along with your intended print size for anyone to advise further.
Oh, and nice shot!
The only explanation possible is that you exported wrong. A RAW file always has the same resolution when it is from the same camera, so at least you won't have to go back and shoot again.
Check your export settings and simply do not apply any Resize and that will output the maximum your camera is capable of capturing. Simply uncheck the Resize to Fit box.
The Image Quality setting has an impact on compression so that reduces the space used by the file. Keep it at 90 or above for a minimal impact on image quality. DPI on the other hand does not matter, it is only when sizing images relative to one-another. By giving the highest resolution possible, the print site will resize as needed for the printer. Do make sure that you are exporting the right aspect-ratio or else your print may get cropped or letter-boxed.
Finally your image is quite dark which means that it has less information than other images taken by the same camera, so compression affects it more, even lossless RAW compression. This does not matter to make a print.