On Debian Buster the file to edit is:

    /usr/share/modsecurity-crs/rules/REQUEST-901-INITIALIZATION.conf

search for: `setvar:'tx.allowed_methods=`

roughly line 163...
edit the line so that it includes the methods you need accept eg:

    setvar:'tx.allowed_methods=DELETE GET HEAD POST PUT OPTIONS'"

save the file then restart apache:
service apache2 restart

acn you are done!