Hi @KellyArmstrong ,
The Join Expression should be a condition that evaluates to true or false (you don't need to return anything). If the condition is met, the features will be joined, if it's not, they won't be joined. Currently your expression is trying to return a result, which isn't the intended use. From the doc here:
Expressions are used in the Join Features tool to specify join conditions.
In some cases, you may want to specify a condition to select features that should be included in the join.
....
A join condition must always result in true or false. Expression examples are included in the sections below.
You will also need to specify $target + $join instead of $feature - to specify which datasets + field you are using. So something like: ($target['media_type'] == 'Curb Tie') && ($join['media_type'] == 'Building Inspector Curb Tie')
But it seems like just applying a filter on your input layer will meet your needs and is simpler, since it sounds like you only want to join features with specific values. You can do this by clicking the "filter" button under the layer in your map, filtering for features where media_type = Curb Tie on one layer, and the other layer has curb type Building Inspector Curb Tie.
Please let me know if you have any follow up questions,
Sarah Ambrose
Product Engineer, GeoAnalytics