Hi,
If someone doesn't want to do something, someone makes an excuse. Awkward.
We often say
If someone doesn't want to do something, they make / he makes an excuse.
If someone doesn't want to do something, makes an excuse. Incorrect
If one doesn't want to do something, one makes an excuse. Fine, but a little formal.
Less formal is
If you don't want to do something, you make an excuse.
In addition, I believe 'one' is not as commonly used in N. America as it is in Britain.
If one doesn't want to do something, makes an excuse. Incorrect
Best wishes, Clive