Yes: It is OK to purchase a candle, as long as you are not spending funds that need to go elsewhere. If you have only been working for five days at your current job, you might not have a whole lot to cover bills otherwise.
If you mean more that you intend to leave a seven-day candle burning constantly, I would recommend against it in general, as leaving a candle unattended is not wise.
In some cases, the ides of using a seven-day candle is to light the candle every day for seven days. In some cases, the idea might be to let the candle burn for seven days, but it is assumed the person doing the working will remain home for seven days, to keep an eye.
There are some people who have given advice in some cases on how to more safely than otherwise leave a seven-day candle burning, but I will not repeat any of the advice, as I will not be held responsible for anything going wrong.