Marriage certificate not available - options?

[softwaretesting]

Hi,

I am applying for TRV for my parents. After I filled out the application online, in supporting documents it is asking to upload marriage certificate. However, we don't have their marriage certificate. I don't remember this requirement from when we applied for their visas 10 years ago.

Has anyone else faced this issue? What are my options now? Is it fine if I just submit their passports which have each other's names as spouses? My birth certificate also has their names.

I searched on this forum for any past post, however could not find anything.

Thanks in advance.