Youtube accept 16:9 ratio of video format. So if you upload greater or less than 16:9 ratio format then Youtube will add black bar while playing.
In adobe premiere pro 2017 if you export video for youtube then you need to adjust setting according to 16:9. In this case, the H.264 format is best for youtube. Just select the preset in this format as 'Youtube 1080p HD'. You could use 'Youtube 480p SD','Youtube 480p SD WIDE' 'Youtube 720p HD'.
Watch this video where I showed how to export video to youtube in premiere pro 2017.
