If you can't seem to enable/show the Story Points field in Jira, take a look at the Context setting.
Jira Issues Settings → Custom Fields → Story Points → click the three dots to bring up the menu → Contexts and default value.
Make sure that the project you want is selected or Global context is selected.
This is also where you can set different issue types to have Story Points field. You can also define multiple contexts.
Story Points and Kanban Board in Jira
I had to add the Story Points field to a Jira project that's been using Kanban board. Surprisingly, Jira doesn't support Story Points field for Kanban boards. One of those "the tool vendor knows better than you" situations, though they are considering it as a future feature request.
There's a value in having estimates for Kanban issues, and it's unfortunate that Jira decided not to support it. At least I can add to the issue detail view, though I wasted some time due to the custom Context setting that this particular Jira Cloud instance was using. It's very possible that it was actually me who initially set it, yes, it's been a while.
Just to make the planning easier for my team, I created a new board based on Scrum, so we can see Story Points in the backlog list view.
Epilogue
This is a bit of a rant really. Jira has a lot of customization features, with schemes and screens and such, though it's puzzling as to why they couldn't just enable Story Points in Kanban boards since they have it in Scrum boards. Like many software implementations that need to evolve, it's interesting to see how Jira is adding new features and keeping existing features on a web product that can't go down and need to service both long time existing users and attract brand new users. First was the New Issue View that you can enable from the personal settings. It lacked a lot of features when it first came out, such as copy-and-paste for images, but now it has implemented enough features that I usually leave it on. Then there's the Next-Gen projects, which currently lacks enough features that it's not usable for me, but probably geared for the new people who are not accustomed to somewhat steep learning curve of Jira configuration. It also doesn't help that built-in fields are configured through a feature named "Custom Fields".
Also, why are there two fields — Story Points and Story points estimation? Well, here's the answer — the first is used for classic projects, and the latter is used for Next-Gen projects. Now I'm beginning to wonder if Atlassian has it all together...is it time to leave Jira and look for something better?
