TroubleShooting
Common
Input string was not in the correct format.
Type mismatching error. Check the types defined in the data-class and compare it with the one specified on the Inspector view.
Excel
'Can't read content types part !' error on Mac
On a Mac machine, opening .xlsx file cause an error 'Can't read content types part !'.
Save it as '.xls' then open again. It solves the problem.
Supported version of '.xls' file
Excel versions 97/2000/XP/2003 are suppported for '.xls' (due to that NPOI, an open source project used to read excel spreadsheet for Unity-QuickSheet does not support Excel versions 5.0/95 for '.xls') But you don't care about it for '.xlsx'.
Deserialize Exception error
Excel Deserialize Exception: Object reference not set to an instance of an object.
There can be an exception error on Unity console such as 'Excel Deserialize Exception: Object reference not set to an instance of an objectRow[5], Cell[6] Is that cejjll empty?' if a cell of a spreadsheet is empty.
Check the cell with the given row and cell index of the error. Then see the cell is defined as string type and not being empty. If the cell is empty fill a proper data.
Value too large or too small out of the range with the given type.
This error is for one of a value is out of range. For an example, long type can has a value ranged [- 9223372036854775807, 9223372036854775807]. If the value in the cell is smaller or later than the range, the above exception is thrown.
Invalid Credential Error
If you met an error which is shown as an invalid credentials when you try to get data by clicking 'download' button, check that your google accout page and you have two-stage verification.
If you have Google two-stage verification on, then it doesn't matter what your Google password is, it won't be accepted. You need to generate (on Google) what is called an Application Specific Password (ASP). Go to Google Account Page and set up an ASP, enter the password you generate as the password in your code, and you're done.
Security Error
Google Spreadsheet plugin does not work in the Unity web player's security sandbox. You should change the Platform to 'Stand Alone' or something else such as 'iOS' or 'Android' platform in the Build Setting.
Underscore character in Column-Header
DO NOT contain underscore character in a column-header.
Google spreadsheet API fails to get values of a cell if the column header of the cell contains underscore.