I ran into this same issue while following the custom text classification quick start guide. I got around the issue by first creating my project in Language Studio with no mapping file (I selected "No, I need to tag my files as part of this project."). Then I used the REST API to add the mapping file. Within the body of the request, you can import the JSON file. I found that the keys need to match the example in the assets object or else I received an error.
"assets": {
"classes": [
{
"category": "string"
}
],
"entities": [
{
"category": "string"
}
],
"documents": [
{
"location": "string",
"language": "string",
"class": {
"category": "string"
},
"classes": [
{
"category": "string"
}
],
"entities": [
{
"regionOffset": 0,
"regionLength": 0,
"labels": [
{
"category": "string",
"offset": 0,
"length": 0
}
]
}
],
"dataset": "string"
}
]
}