Enable SPD 2013 under Group O365 Site

[Deleted]

I tried to open my new Group O365 Site with SPD 2013 but i get this error msg: "403 FORBIDDEN403 FORBIDDEN403 FORBIDDEN403 FORBIDDEN403 FORBIDDEN" I understand that i should enable SharePoint designer for this site but i couldn't find "SharePoint Designer Settings" under Site settings->Site Collection Administrator.

[#2]

And what do you want to do with SPD 2013 in a modern SPO Site :-)?

[#3]

To work with Workflows. 

[#4]

I got the answer from here: https://support.office.com/en-us/article/Allow-or-prevent-custom-script-1f2c515f-5d7e-448a-9fd7-835d...

now i'm able to work with SPD in my Group O365 Site.

[#5]

I have a group site in SP 365, it is having a 3 days pending task remainder mail notification, where should i find it to update it?