Putexcel modify don't work as intended

I have made a Gantt chart for project management where I want to import date variables from a Stata data file. My thought was to this with the put excel command in Stata, but it seems like the "Modify" option doesn't work?

My predefined Gantt chart is placed in my change directory and I have written the below code for testing the modify option:

putexcel set Gantt_diagram.xlsx, sheet("Sheet1") modify
putexcel A1 = "Test"
putexcel A2 = "Test2"

Stata seems to run the code fine, but there is no output in A1 or A2??
Isn't it possible to modify a predefined excel file that has not been created in Stata?

Kind regards

Frank