I could easily count to 1 billion, but I don't have the time to undertake that task.

Sibling comments are missing the point.

I am capable of make video game A. I am also capable of making video game B.

I do not have the capacity to make both video games in parallel, even if I give myself twice the time. A core level of attention and care is required by each, of which I only have capacity to focus on one at a time.

Example: I am physically/mentally/financially capable of taking a bike tour across US, to do that I need X amount of time.

Owing to other responsibilities I can not allocate X amount of time right now. (Ignoring whole COVID-19 saga of course)

As the other comments illustrate capability pertains to something within the person, and capacity pertains to factors outside the persons control.

Hence the person has the capability to count to 1 billion, but not the capacity to do it because other obligations will prevent them taking the required time.

Of course 1 billion seconds is a little over 30 years, so probably the external capacity limit on that task is mortality, dependent on starting point.

You've got 100 Terabytes of storage in the cloud (capability) but only a 100 Mbps connection (capacity).