Unable to use aggregate fucntions in Text on Symbol expressions

[toasthall]

When trying to use aggregate in expressions to set labels for Text on Symbols I get the following error:

QGIS and OS versions

QGIS version	3.10.0-A Coruña	QGIS code revision	6c816b4
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	2.4.1	Running against GDAL/OGR	2.4.1
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
Compiled against SQLite	3.29.0	Running against SQLite	3.29.0
PostgreSQL Client Version	11.5	SpatiaLite Version	4.3.0
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	5.2.0	Running against PROJ	Rel. 5.2.0, September 15th, 2018
OS Version	Windows 10 (10.0)
Active python plugins	AttributeAssignment; autoSaver; CalcArea; CalculateGeometry; DigitizingTools; FreehandRasterGeoreferencer; latlontools; qgis2web; quick_map_services; scriptrunner3; SemiAutomaticClassificationPlugin; SpreadsheetLayers; track_profile_2_web; db_manager; MetaSearch; processing

[gioman]

Maybe it is by design, but not 100% sure at the moment.

[roya0045]

That's normal, I have PRs that would solve this but they are in limbo for the moment. See #30727

Edit: the PR would enable this in the layout legend instead of the symbology as you are trying to do. As of now the symbology expression doesn't have access to the symbol context. This should be a trivial change but you still wouldn't be able to do aggregate operations based on the symbology until my PR is merged.