Errrr It's abit confusing at first and I have to open your error_log to find out what caused the error and found that the error is because you entered 'auto' in the width and height of thumbnail settings
The width and height of thumbnail must in numeric number. It will be resize automatically by css so you don't need to add 'auto' there
https://www.dropbox.com/s%2Fk97jtdmkhhfwlvy%2Fmonosnap%25202021-05-02%252012-59-28.png?raw=1