Query (2, 1) Failed to resolve name 'AG'. It is not a valid table, variable, or function name.

Hi Anonymous,
Thank you for reaching out to the Microsoft fabric community forum. Thank you Deku, bhanu_gautam, for your inputs on this issue.

After thoroughly reviewing the details you provided, I was able to reproduce the scenario, and it worked on my end. I have used it as sample data on my end and successfully implemented it.    

Dax Measure for UAM Aggregated:
UAM_Aggregated =

ADDCOLUMNS(

    SUMMARIZE('UAM', 'UAM'[UG1]),

    "AG_Combined", CONCATENATEX(

        FILTER('UAM', 'UAM'[UG1] = EARLIER('UAM'[UG1])),

        'UAM'[AG],

        ";"

    )

)

I am also including .pbix file for your better understanding, please have a look into it:

If this post helps, then please give us ‘Kudos’ and consider Accept it as a solution to help the other members find it more quickly.

Thank you for using Microsoft Community Forum.

Anonymous , Use 

dax
AG =
CONCATENATEX(
FILTER(
'UAM',
'UAM'[AG] = MAXX(ALL('UAM'), 'UAM'[AG])
),
'UAM'[UG1],
";"
)

When using EVALUATE the output needs to be a table

You could something like

Evaluate

Define AG =CONCATENATEX(  

        FILTER(

            'UAM',

            'UAM'[AG] = MAX('UAM'[AG])

        ),

        'UAM'[UG1],

        ";"

    )

{Ag}