I would submit the paper to get the process started, but send a note to the editor that Prof. X has not yet given permission to be named, though you have tried to contact him. The editor will make a decision. Perhaps the paper will be returned to you. Perhaps the decision to publish will be deferred, but review begun. Perhaps the editor will ask the person if they will permit. There are a lot of possibilities.

But in April 2020 the world of academia is in chaos due to a worldwide pandemic. Lots of things that used to take a while, not take a very long time. And some people are dealing with illness, their own or that of family members. Don't expect communication to be quick at the present time.

You could also explain this to the professor yourself, that you have submitted, provisionally, but will respect his decision. You can always modify the ack later if needed. The review process will take plenty of time.

I would submit the paper to get the process started, but send a note to the editor that Prof. X has not yet given permission to be named, though you have tried to contact him. The editor will make a decision. Perhaps the paper will be returned to you. Perhaps the decision to publish will be deferred, but review begun. Perhaps the editor will ask the person if they will permit. There are a lot of possibilities.

But in April 2020 the world of academia is in chaos due to a worldwide pandemic. Lots of things that used to take a while, now take a very long time. And some people are dealing with illness, their own or that of family members. Don't expect communication to be quick at the present time.

You could also explain this to the professor yourself, that you have submitted, provisionally, but will respect his decision. You can always modify the ack later if needed. The review process will take plenty of time.