Bring your finger close to the camera and you will see that the flash icon is disabled as you mentioned. But at the same time you will see two yellow dots ( focus enhancer) which pop on the bottom left (of phone is in portrait mode) Press them to turn it off (they will go white ) That's it.. now you can use the flash close up.
Why use flash though? Won't it make the documents shine or have uneven light? I am sure with scene optimiser you would get better photos without flash.
Paper is on a table. Light source is above. I never had this issue with my s10 as the flash "just worked". The work around above may work for me though.