@Priyankha We need a lot more details.

FOG Version

What bootfile is your boot server trying to send?

Is you server sending the right FOG server for option 66/67?

What OS are you trying to capture?

What does the error show or maybe a screen shot? (UEFI Boot looks different from Legacy boot from Mac from arm, etc…)

What type of machine are you trying to image?

What have you tried?