@Quincy @Airbnb Thanks for your help. Just to update, I was advised by my CS to try submit the claim by selecting 'Trip related issue' and doing it that way. THAT did submit the claim, albeit the level of detail required is not the same as the option I tried earlier. It only requires amount of claim, and ability to upload docs / photos / receipts and a message to the guest; a lot simpler.
My claim is for 'Damage or missing items...', but there appears to be a technical issue when I select that option. Apparently, when adding an item for the claim, I am supposed to see other fields when scrolling down, but these are not visible, despite trying on various alternative browsers and different pcs, even with deleting caches, history, cookies, etc.
As it seems someone else earlier also made some comment about these fields not being visible suggests airbnb should be aware of this bug, and yet appear to have not resolved it so far! Not sure why it appears only some are experiencing this issue.