have you disabled the field?
sm31> V_PTARQ_TPROCESS?
the PTARQ - Approver (program RPTREQAPPRCHK) , can you verify this?
share your screenshots?
In order for you to assign the approver as you wish, please use the
BADI PT_GET_REQ method FIND_RESP_AND_DEFAULT_NEXT_PRC.
To enable the application to fill up the next approver in case he is
absent or the position is vacant , you can implement method
CHECK_IF_ACTOR_ABSENT of Badi IF_EX_PT_GEN_REQ.
You can define your logic to fill up this position in this method.