Noob here struggling with a formula

I am trying to do a formula that combines a Vlookup, ISNA, IF & AND functions but I am really struggling, I am wondering if anyone is able to help or offer any suggestions? Sorry if this is a silly question but I am new to all of this.

So Basically, if the value of A2 is "Pending Prepayment" and the formula cannot find the value of C2 within '[Finance & Order Value Queries.xlsx]Proforma Orders'!$C:$C then I would like the formula to say "Error" but if the value of A2 is anything but Pending Prepayment I don't need it to do this check so I would like it to say "OK"

=IF(AND(ISNA(VLOOKUP(C2;'[Finance & Order Value Queries.xlsx]Proforma Orders'!$C:$C;A2="Pending Prepayment";FALSE)); TRUE);"ERROR";"OK")

Any help is very much appreciated!

Many Thanks,

Kind Regards,