Well as written in my post to you,
the model and year of your mac will determine which OS you can upgrade to.
That is on the fourth line.
Then you are asked to click on the links below,
in each Section 1 of these is the Compatibility table,
this describes which macs can upgrade to the respective
OS's. If for example you cannot upgrade to Mojave, look
at High Sierra or Sierra, if your mac does not meet the criteria
for the OS then your mac cannot upgrade any further.
When you made your post asking for help you did so hoping that people would
read your post and give information relevant to your enquiry.
In respect of those that take the time to answer your question, given the fact that you did not
bother to give any details regarding the model and age of your mac then the response given
to you can only be a general response.
It is then incumbent on you to read the details of the response in respect of Section 1 in each
of the How To Guides I posted above. Had you read those thoroughly then you would have
noticed that you would not have been able to upgrade your mac further.
If you had given the relevant details of your mac in respect of model and year you would have got
a direct response that your mac cannot upgrade any further.
Now knowing that you cannot upgrade further on your then you would need to buy a pre-owned mac
that is capable of running Sierra and High Sierra or perhaps Mojave. Or buy a new mac which will come
with macOS Catalina.