Analysis of the problem: It may be that the client software of ORACLE is installed, and you only configure the service under the ORACLE installation program, but do not configure the service under the ORACLE client installation program.
solution:
1. Open the Net Configuration Assistant under the oracle client. If there is no monitor, you need to create a monitor. If you already have one, you don't need to create it again.
2. Open the Net Manager under the oracle client and create a service. The method of creation is the same as that under the oracle software.
3. Open PL/SQL again, and you will find that there is one more database you just created in the drop-down list of the database.