When creating a new Variable, you generally will not have to choose a qualification, as ETO Results will choose the correct one for you.
There are 3 possible qualifications for a Formula:
A Dimension of a piece of data. It may contain a single value (like a Demographic for a Participant) or a set of values (like Program Start Dates for a Participant.)
A Measure is an Aggregate (Sum, Count) or Limiting (Max, Last) of a Dimension. It will always represent a single value. This is often an Aggregate of other report Objects, but not always. It can also be a "paring down" of an Object to connect to another Object in a specific way.
There are some instances where Variables are created as Measures when they do not need to be. A Variable only needs to be a Measure if it is performing an Aggregate function.
A Detail is a sub-set of other Objects in a report. It represents a smaller piece of a Dimension. A Detail Object must belong to a Dimension Object and has a 1-to-1 relationship with it.
Some Flattened Objects will come with Detail Objects, such as "Name" (pictured below). You can find these within Flattened Data folders. The root object will have a + next to it.
A Detail needs to be attached to an associated Dimension, usually a Merged Dimension across multiple Queries.
If you receive the following error upon creating or editing a Variable, simply change the Qualification from a "Dimension" to a "Measure". That should resolve the error.