Cannot SUM

So happy to find this plugin!

I am not able create a sum. I use =SUM(A4;A9) for example, and all I see in the sell is exactly that, but no sum.
Am I missing something?

I also do not see the F(x) on my spreadsheet next to Save and Cancel.

Thanks for any help.

https://www.ads-software.com/extend/plugins/dhtmlxspreadsheet/