Job Listed "Clearance: Not Required" on Announcment. Now A Requirement?

Hello. I accepted a job. There was no clearance requirement listed on the USAJOBS posting.

Now, after being on the job for some time, I am being told by my leadership that I need a fancy Top Secret clearance to touch any of the work. Can the agency now require this if it wasn't listed on the USAJOBS posting as a requirement?

I want to do good work, and I want to contribute. But I will not be approved for a clearance, and I do not want to have to report a previous clearance denial when asked in the future.

Will I just lose my job if I don't get the clearance? Do I have any options here?

I appreciate any thoughts, spitballing, opinions, or expert knowledge.

I have a family to support, and am feeling the dread of potential job loss hanging over me.

Thank you.