Could not find artifact mysql:mysql-connector-java:pom:unknown in nexus-aliyun

This error message indicates that Maven is unable to download the mysql-connector-java artifact from the repository specified in your POM file. The "unknown" in the message suggests that Maven is unable to determine the version of the artifact to download.

Here are some potential solutions:

1. Check if the artifact is available in the repository: Go to the repository browser and search for the artifact. If it is not available, you may need to add the repository that contains the artifact to your POM file.

2. Check if the version is correct: Make sure that the version of the artifact specified in your POM file is correct. You could try to find the latest version of the artifact and update your POM file accordingly.

3. Check your Maven settings: Check if your Maven settings.xml file has the correct repository credentials and configurations to download the artifact.

4. Force an update: Try to force Maven to update the artifact by adding -U to your Maven command. For example, "mvn clean install -U".

If none of these solutions work, you may need to contact the repository manager or the artifact owner for further assistance.