SP03 The field string-"" must me an alphabetical string (only letters)

The field string-"" must me an alphabetical string (only letters)

I faced this error during drill down of chart 1 to 2nd chart.

I created an analytical driver of type = "string"  and same driver is being used by chart properties.

Below is the picture of analytical properties , where you can see under "predefined values constrains"
"Letter" is checked

Which is causing to populate this error. Here we are actually add consrtaint on string type which will check that string should contain only "LETTER"

How to Resolve:
Go to Profiling - Analytical driver management - identify the driver from chart properties and edit the analytical driver
remove the constraints