Looking on the internet deeply has found these results:

I am not certain on what I am supposed to do. is the most popular phrase on the web.

I am not certain of what I am supposed to do. 11 results on the web	I am not certain on what I am supposed to do. 1288,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Captain, I am not certain I fully understand this Prospero character. I am not certain that is really a satisfactory answer. I am not certain, sorry. I am not certain that human or Bajoran rites are fitting. I am not certain if they primarily serve Europe's interests. I believe that his family have succeeded in bringing him essential medicine, but I am not certain of this. Mr Schulz, I am not certain we can discuss this point again. I am not certain whether you are responsible for this area, so I would ask you to pass it on if necessary.	I am not certain whether you are responsible for this area, so I would ask you to pass it on if necessary. I am not certain that human or Bajoran rites are fitting. I believe that his family have succeeded in bringing him essential medicine, but I am not certain of this. Mr Schulz, I am not certain we can discuss this point again. I am not certain that is really a satisfactory answer. Captain, I am not certain I fully understand this Prospero character. I am not certain if they primarily serve Europe's interests. I am not certain, sorry.

I am not certain of what I am supposed to do.

11 results on the web

I am not certain on what I am supposed to do.

1288,000,000 results on the web

More popular!

Captain, I am not certain I fully understand this Prospero character.
I am not certain that is really a satisfactory answer.
I am not certain, sorry.
I am not certain that human or Bajoran rites are fitting.
I am not certain if they primarily serve Europe's interests.
I believe that his family have succeeded in bringing him essential medicine, but I am not certain of this.
Mr Schulz, I am not certain we can discuss this point again.
I am not certain whether you are responsible for this area, so I would ask you to pass it on if necessary.

I am not certain whether you are responsible for this area, so I would ask you to pass it on if necessary.
I am not certain that human or Bajoran rites are fitting.
I believe that his family have succeeded in bringing him essential medicine, but I am not certain of this.
Mr Schulz, I am not certain we can discuss this point again.
I am not certain that is really a satisfactory answer.
Captain, I am not certain I fully understand this Prospero character.
I am not certain if they primarily serve Europe's interests.
I am not certain, sorry.