Automatically replace dots with commas in a Google Sheets Column with Google Script

• Select the column you want to change.
• Goto Edit>Find and Replace

• In Find area put "."

• in Replace with area put ","

The error occurs because .replace is a string method and can't be applied to numbers. A simple workaround would be to ensure the argument is always a string, there is a .toString() method for that.

in your code try

return [row[0].toString().replace(".", ",")];