How do I find my payers federal id number

It's  the form sent to you by the entity that paid you. Where exactly on that form, depends on the form you were sent. Generally, it's in a box labeled something like "Federal Employer Identification Number".

What form are you trying to enter?  There is no EIN on social security SSA-1099.  If you got a SSA-1099 you are entering it in the wrong place.