If you are on Windows 64-bits, be sure to check this issue, and the workaround of deleting/renaming from the Processing IDE folder this file: \processing-3.x.x\modes\java\libraries\serial\library\windows32\jSSC-2.8.dll
If you are on Windows 64-bits, be sure to check this issue, and the workaround of deleting/renaming from the Processing IDE folder this file: \processing-3.x.x\modes\java\libraries\serial\library\windows32\jSSC-2.8.dll