CellType is in one of the dependencies pulled in by Gradle, not in my source or the JDK 11.0.5.
I think you may need to use Build->Gradle Options to pull it down. Be careful though, Build->Gradle Options overwrites the vpj file so you need to put back the old vpj file after you do that (Patrick has an upcoming fix for that vpj overwriting issue -
https://community.slickedit.com/index.php/topic,17345.0.html).
On my machine it is at: C:\Users\Rob\.gradle\caches\modules-2\files-2.1\org.apache.poi\poi\4.1.1\45d063946e80a2d08a209bebc7d87c7ad8d6d2cb\poi-4.1.1-sources.jar\org\apache\poi\ss\usermodel\CellType.java
From the build.gradle, it is from one of these 2:
// https://mvnrepository.com/artifact/org.apache.poi/poi
implementation group: 'org.apache.poi', name: 'poi', version: '4.1.1'
// https://mvnrepository.com/artifact/org.apache.poi/poi-ooxml
implementation group: 'org.apache.poi', name: 'poi-ooxml', version: '4.1.1'