F5 gateways failed to discovered through their IP address resulting in the pop-up error message of "Invalid credentials".
The F5 gateways' engineers confirmed that the connection is okay and successfully authenticated with SNMP v.2c.
Release : 20.4
snmpcollector 4.12
As the F5 vendor's engineer clarified, finally F5 servers are not supporting SNMP.V3 (SHA256/AES256). The discovery was successful with SNMP.V3 (SHA128/AES128).