Cannot Choose Disneysea as Valid Park?

Hi there, I'm experiencing problems in booking my 1-day passport for Disneysea online. The only option available for the 1st day park was Disneyland. I went ahead to buy the tickets hoping that I could edit my booking. In the amendment page, I tried to change the valid park to Disneysea but an error appeared stating that I cannot choose Disneysea as the valid park instead. Has anyone faced this problem before? Could someone advise?

Thank you!!

If tickets for a park are sold out for the day you want then you cannot buy them online. They will not show up as an option. TDS is probably sold out while TDL still has room for more guests.

Since there are no refunds for online purchases, all you can do is go to TDL on the day you have tickets, or change the date to one where TDS tickets are not sold out, or continue checking to see if TDS tickets become available on the day you want. You can make changes without cost.

This is all stated clearly on the TDR online ticketing web site.