i couldnt create an account in mercari until i tried during checkout

for whatever reason it always gave me an error saying you cannot register as a member when i put my phone number but once i tried checking out and filled out everything an account was created and i could make orders

this is on japan site version. now that i have an account im just confused as to why i couldnt create an account on the app or website during sign up. oh it also tells me to verify my phone number which i still cant for whatever reason