Welcome to the forum!
I keep this statement ready for people looking for their pokestop:
I know the prompt tells you that you are submitting a “Pokestop” but you are actually submitting a Wayspot to the Lightship database, which can only become a Pokestop or gym if it is alone in a level 17 S2 cell. This is a good article about S2 cells and PoGo: https://pokemongohub.net/post/article/comprehensive-guide-s2-cells-pokemon-go/
If you would like to leave the gps coordinates, someone can check and see if this is the issue.