Datatable row add value

It is not possible to put strings into decimal columns on a datatable.
If you really want to vary what is stored in this way then change the datatable definition to typeof(string) and store any numbers using .ToString()

Example:

DataTable dt = new DataTable();
dt.Columns.Add("col1", typeof(string));
dt.Rows.Add((1.2).ToString());
Debug.Print(dt.Rows[0]["col1"].ToString());
dt.Rows.Add("a");
Debug.Print(dt.Rows[1]["col1"].ToString());

produces the output

1.2
a

It's a question of what are you actually trying to achieve here

The thing is you can't just add a string to a column that was made to accept decimal values. Why don't you instead change the column type to string for simplicity. If it is a decimal, then the column will implicitly convert it to string. If you enter a string it will accept it as well.

I just answered a similar question where I added two columns in a DataTable and added two rows. Look at my example, I needed to display date but I kept the DataTable column type string for simplicity where as the variable I used to get the Date was a DateTime variable: Datatable two line columns[^]