Exported video to mp4 off Premiere Rush but the file can't be uploaded on any sites?

Hi! This is my first day using Premiere Rush and it's been great. I made a video around 3 minutes long and exported it as mp4 no problem. The file is very big.. around 300MB. Anyways, I tried to upload it to Youbtube, Vimeo, among other sites and it simply can't be processed. It doesn't open with Quicktime either. Any suggestions? Is the file somehow corrupt? I don't want to play around with exporting since there's only 3 free exports. Thanks!