I am using this endpoint https://developer.atlassian.com/cloud/confluence/rest/v1/api-group-content-labels/#api-wiki-rest-api-content-id-label-post
and passing the pageid as content id. (the one we can see in the URL , something like this /pages/445654724/)
I am the space admin for the space in which this page is, I am also the creator of this page. Somehow it gives me 404 
Well, that would explain the 404 error, since that API endpoint you provided a link to doesn’t have any such ‘pages’ URL path.
Yes, but there is content/{id}
So, I thought we pass the ID of the page here.
SLB-Private
You do pass the ID of the page there, just like it says in the documentation… but you said you were using pages/{id} in the path URL, which is an invalid path. If you are now saying you are using content/{id} in the path URL, then that is a valid path and should work.
You need to provide a COMPLETE description of the problem. Refer to the How to ask a good question thread for a better understanding of the minimum level of information you need to provide.
And I have no idea whatsoever what ‘SLB-Private’ means.