Also ran into the missing button "Continue to Payment" yesterday applying GPUs.
Outbound with 3 segments and return with 4.

Contacted Twitter support and had to receive a call from the Reservations team to answer security questions and apply the GPUs.

Then I asked the agent on Twitter if there was a specific reason or if this was a bug and was told:
"That could be a glitch on the website and we apologize for the inconvenience caused..."