Kevin Timpe distinguishes between the concepts of free will and free action. He writes, "We most often think that an agent’s free actions are those actions that she does as a result of exercising her free will." And,
If we assume that human actions are those actions that result from the rational capacities of humans, we then see that the possibility of free action depends on the possibility of free will: to say that an agent acted freely is minimally to say that the agent was successful in carrying out a free volition or choice.
Although free will and free action are related he cautions that "it is important not to conflate them":
However, one might still believe this approach fails to make an important distinction between these two related, but conceptually distinct, kinds of freedom: freedom of will versus freedom of action. This distinction is motivated by the apparent fact that agents can possess free will without also having freedom of action.
In the case of the example provided by the OP where someone goes to the store intending to get tape but forgets to do so, free will was exercised in the choice to get tape. Free action was constrained when the person forgot to actually do so.
With that preliminary, let's consider the questions:
Why was I unable to do what I wanted to do, despite nothing physically stopping me?
Sometimes we forget. Memory is then a constraint on our free action.
Is there a relationship between memory and free will?
In some situations there may be more of a relationship between memory and free action than memory and free will.
Kevin Timpe, "Free Will", Internet Encyclopedia of Philosophy https://www.iep.utm.edu/freewill/