dexcom g6

I just got the apple watch series 7 last night. the update is 9.5. I am a type one diabetic and needs the dexcom g6 app. when i try to get it it says it requires 13.0 watch os. ok fine, i get follow for dexcom. it installs, but when i try to open the app it says “to use this app on your apple watch, the iphone app follow needs to be installed on (null)” but i need it to work for when i go back to school and nobody knows how to fix it