What is "ACO transition"?
A transition is a branch of a Standard Terminal Arrival Route (STAR) or a Standard Instrument Departure (SID). ACO is the identifier for the Akron VOR/DME, which is a a navaid. It’s not surprising then that you would hear of an ACO transition for both KCAK and KCLE.
I would provide a chart showing where these are, but I can’t find any plates for the older VOR navigation. All the ones I find are RNAV which use GPS rather than navaids. Although the ACO VOR/DME is still operational, they are slowly being decommissioned. The FAA may have eliminated the VOR SIDS and STARs and are now only using the RNAV routes. It’s likely that the sim is using outdated plates.
If you are unfamiliar with SIDs and STARs you can check out This question.