Azure Data Factory 'Failed Validation' with Folder does not exist
I had wrong file extension.  Error message was misleading. Instead of cannot find file I thought the error was about missing folder.

I had to change this

 "typeProperties": {
            "fileName": "{Day}.csv",
            "folderPath": "adf-data/sales/{Year}/{Month}",
            "format": {
                "type": "TextFormat",
                "columnDelimiter": "\t",
                "firstRowAsHeader": true
            },


To (change file extension from csv to txt)

"typeProperties": {
            "fileName": "{Day}.txt",
            "folderPath": "adf-data/sales/{Year}/{Month}",
            "format": {
                "type": "TextFormat",
                "columnDelimiter": "\t",
                "firstRowAsHeader": true
            },