How about trying to OLCI just your partner? So using a clear browser, go to
ba.com/checkin
then use the last name of the companion and the PNR, don't do your check-in and see what happens.

API isn't used for all countries, but it is for HKG and Canada, though not SIN (trying to think of the routes!). If it is Canada then there is an ETA requirement for example. The online management of API under the BA profile did fall over a few days ago which may or may not have been related to the Pre-Check KTN issue.