Hi Arianna, if I understand the question correctly, you are trying to change the units of the inputs/outputs that are not the quantitative reference (so the ones, you did not create yourself)
In openLCA the flows you use in inputs/outputs have a list of flow properties. Usually only one flow property is specified in the reference data. Each flow property has a reference to a unit group, which respectively has a list of units, that can be converted within this group (e.g. 1 kg = 1000 g).
You can only select those units referenced via the "Flow -> Flow property - Unit group" chain.
If you want to select a different unit, you would need to add the correct flow property (e.g. Mass) to the flow's flow properties list and specify a conversion factor to the other flow property.
An example: You have the flow 'Methanol' an its set flow property is Volume. You want to specify Methanol in kg. The density of Methanol is 0.79 g/cm³. So to convert 1 l of Methanol to kg, you will need to use a conversion factor 0.79 for conversion. Now you could use all Mass and Volume units to specify your inputs/outputs.
I hope this helps. Below you can see how this would be done in openLCA (Just as an example, this is not part of the reference data)