@kfavara ,
Two things jump out at me.
The documentation for enable_sis_reactivation says
When true, will first try to re-activate a deleted section with matching sis_section_id if possible.
Notice the non-committal language there? "try" and "if possible". Maybe it isn't possible to reactivate the section you're trying to re-activate.
The second is what you're trying to send. I didn't test what you said you're sending, so I can't say for sure that it won't work. However, I tested what the documentation says to send and it worked for me and what you're sending doesn't match the documentation I'm looking at so maybe we're looking in two places.
The URL you're using matches the Create course section endpoint of the Sections API. It's the same URL for Listing the sections, with a GET, but Create with a POST. You definitely need to be doing a POST.
I created a course by POSTing the following JSON to /api/v1/courses/2151246/sections
{
"course_section":{
"name":"test section",
"sis_section_id":"m113beta-test01"
},
}
It returned this object

I then deleted it by sending a DELETE to /api/v1/sections/2416439
The section was deleted form my course.
Now I try to create the section again by doing a POST to /api/v1/courses/2151246/sections
This time the payload is
{
"course_section":{
"sis_section_id":"m113beta-test01"
},
"enable_sis_reactivation":true
}
And my section is back with the same Canvas section ID (2416439) that it had to start with.