FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
When adding a SNMP sensor in the PRTG tool for interface monitor.
When entered the OID it returns the below output:

This article describes how to fix this error.

First, understand what the OID means.

Example: OID:

The value 16 is the value of 'snmp-index' of an interface.
the method GetNext is used, which means requesting for the next interface value which will be 'interface with 'snmp-index 17'.

Output for reference.
edit "Rel PRI-P2P"
    set vdom "root"
    set ip 172.17.x.x 255.255.x.x
    set allowaccess ping https
    set status down
    set snmp-index 17
    set interface "port2"
    set vlanid 20
To fix the issue, check if there is no other interface with same SNMP index value.
If there is another interface with same 'SNMP-index' just change it to something else and try creating the sensor again from PRTG tool.

Commands to change the 'SNMP-index'.
# config system interface
    edit <interface name>
        set snmp-index <value>