thumbnail creation failed

the thumbnails are not generated and I have only PDF files

error:

[07-Nov-2020 02:55:37 America/Chicago] PHP Fatal error:  Uncaught Error: Undefined class constant 'ALPHACHANNEL_REMOVE' in /home3/hashim/apad.lk/library/system/classes/vendor/FileRun/Thumbs/Resizers/IMagick.php:38
Stack trace:
#0 /home3/hashim/apad.lk/library/system/classes/vendor/FileRun/Thumbs/Resize.php(14): FileRun\Thumbs\Resizers\IMagick::resize('/home3/hashim/a...', '/home3/hashim/a...', Array)
#1 /home3/hashim/apad.lk/library/system/classes/vendor/FileRun/Thumbs/Make.php(144): FileRun\Thumbs\Resize::resize('/home3/hashim/a...', '/home3/hashim/a...', Array)
#2 /home3/hashim/apad.lk/library/system/classes/vendor/FileRun/Files/Actions/Thumbnail.php(136): FileRun\Thumbs\Make::make(Array, Array, true)
#3 /home3/hashim/apad.lk/library/system/modules/fileman/sections/get/php/thumbnail.php(27): FileRun\Files\Actions\Thumbnail::show(Array, Array)
#4 /home3/hashim/apad.lk/library/index.php(191): unknown()
#5 /home3/hashim/apad.lk/library/t.php(6): unknown()
#6 {main}