First, I am sorry you are having issues, you sound frustrated.
That said, simply saying you "have followed all the steps correctly" is not helpful. To me, at least, it sounds slightly aggressive, and you're straightaway cutting off any possibility the solution could be found on your end when, statistically speaking, that's the most likely place for it to be.
Let's try this though and see if we can get some more helpful information.
Very first thing. Look in the upper right-hand corner of your R Studio application. You should see something that identifies the project you are in. If it says Project: (None) that's the problem, you're not working in a defined project, so there's nothing to version control.
If there is a project name listed there, that project is likely not set up in your local git or there is some other git issue. But, let's try something simple to attempt to isolate the problem.
Click on File/New Project, select the New Directory under Create Project, then select New Project under Project Type.
Give the directory a name, doesn't matter what it is, we're just exploring here.
Then, under the text area where the parent directory is listed in the same modal window, make sure the checkbox next to Create a git repository is checked.
Finally, click on the Create Project button.
Now, see if you have a git tab in the Environment pane.