Hi. I finally found a solution.
I was stuck with a grayed out 'CPNY FLT-PLN REQUEST' button with an aircraft loaded from the saved misson on MSFS 2020.
I tried to start a brand new fresh mission instead with the same origin and arrival, reloaded my SimBrief plan in OIS and this time the 'CPNT FLT-PLN REQUEST' button was active on the FMS INIT page.
I guess this had something to do with the saved mission. But this workaround was successful. The source of the issue is still TBD though...