You'd need to save the layer with the join so that the attributes are "baked in".
The symbology profile in Arcade does not support referencing other layers, or even other features in the same layer, so you need to be able to access the data within the feature's own attributes.
EDIT: There is another way: hosted layer views. You can perform an attribute join in the web map, then publish the results as a hosted view layer. This will add the attributes from the table as though they were "baked in", but still references the original two datasets, and remains up to date as edits are made to the underlying data.
Thanks for the response, here my issue is how do we get joined attributes symbology in feature service when published in ArcGIS online. Please find below variations from ArcMap to Published service symbology. Is there any way to get the same symbology define in ArcMap?
ArcGIS Online published Service
To put it simply, you don't. At least, not initially.
You'd need to export the layer and make the join permanent, then publish that, or you'd need to publish the layers as-is and establish a hosted layer view after the fact. There's no way to use a joined table's symbology in AGOL with the layers being separate.