Sorry for my reply. I am only an F-Secure user (their home solutions).
I am not sure if there is available F-Secure SAFE package for two units only (I do able to suspect that it can be "up to three units" or "one unit"; but maybe your experience is about customized offer).
Anyway, did you mean that licence is marked as expired or invalid during try to install OR for existed installation?
One option is to login into your My F-Secure Account (my.f-secure.com). To doublecheck that subscription is about two units. And then to trigger fresh installation for your second device (Mac). Maybe, currently, second device installation with broken state (licence/device is removed from Portal, for example); or device installation is not pinned to your subscription.
Maybe it is also useful to know what is your Mac build (version OS). What if there are some limitations.
And good to know what is exact output trouble (and where/when "its says").
Another option is to contact their official Support Channels (chat as example):