With a usrmerged kernel, vmlinux(.xz) is in the modules directory for the
kernel:

/usr/lib/modules/5.14.1-1-default/vmlinux.xz

This means the debug info ends up here:

/usr/lib/debug/usr/lib/modules/5.14.1-1-default/vmlinux.debug

Crash only looks at the old location though, and fails:

please wait... (uncompressing /usr/lib/modules/5.14.1-1-default/vmlinux.xz)

readmem: read_diskdump() 
NOTE: gnu_debuglink file: vmlinux.debug
crc32: f73ed3b0
/usr/lib/modules/5.14.1-1-default//vmlinux.debug: not readable/found
/usr/lib/modules/5.14.1-1-default//.debug/vmlinux.debug: not readable/found
/usr/lib/debug/boot/vmlinux.debug: not readable/found
crash: /var/tmp/vmlinux.xz_DZPxr5: no debugging data available
crash: vmlinux.debug: debuginfo file not found

crash: either install the appropriate kernel debuginfo package, or
       copy vmlinux.debug to this machine