Since Release 4.0, The SequenceDiagram Core didn't bundle language module, this is big change from Relealse 3.0. After you install SequenceDiagram Core, you must install atleast one luaguage module before you can using it. For your situation, you just install SequenceDiagram Java from marketplace, the is another plugin, and it's free.

for more product information, SequencDiagram Site

SequenceDiagram Core + SequenceDiagram Java work for you.

Closed inssure do not meen that plugin will be working !!!
#185

Do not think that we are ...... Do you think I will am buy NOT working plugin!

Advace
Install "SequenceDiagram Java" for java/kotlin/Scala language support from Marketplace"
after my message is
"INSTALLED from Marketplace!"
, I do not think that You are want people to buy your not working product!

I guess you probably don't quite understand my answer. You need to install two plugins at the same time to work together, SequenceDiagram Core and SequenceDiagram Java. This is designed to support more languages. I'm sorry for the trouble, I hope you understand.

Short answer and solution for you:
After you install SequenceDiagram Core, install SequenceDiagram Java also. Multi plugin work together, this is how Release 4.0 was designed. Multi module support more lauganges: Java, Kotlin, Scala, JavaScript, TypeScript, Python, Rust, Go, C++.

Thanks!
(maybe message must change " unsupported java version" to "not install java plugin") 👍