Você parece estar usando o Busybox find , não o GNU find . Embora o Busybox find deva suportar -delete , a versão com o Ubuntu não parece .
$ busybox find --help
BusyBox v1.21.1 (Ubuntu 1:1.21.0-1ubuntu1) multi-call binary.
Usage: find [PATH]... [OPTIONS] [ACTIONS]
Search for files and perform actions on them.
First failed action stops processing of current file.
Defaults: PATH is current directory, action is '-print'
-follow Follow symlinks
-xdev Don't descend directories on other filesystems
-maxdepth N Descend at most N levels. -maxdepth 0 applies
actions to command line arguments only
-mindepth N Don't act on first N levels
-depth Act on directory *after* traversing it
Actions:
( ACTIONS ) Group actions for -o / -a
! ACT Invert ACT's success/failure
ACT1 [-a] ACT2 If ACT1 fails, stop, else do ACT2
ACT1 -o ACT2 If ACT1 succeeds, stop, else do ACT2
Note: -a has higher priority than -o
-name PATTERN Match file name (w/o directory name) to PATTERN
-iname PATTERN Case insensitive -name
-path PATTERN Match path to PATTERN
-ipath PATTERN Case insensitive -path
-regex PATTERN Match path to regex PATTERN
-type X File type is X (one of: f,d,l,b,c,...)
-perm MASK At least one mask bit (+MASK), all bits (-MASK),
or exactly MASK bits are set in file's mode
-mtime DAYS mtime is greater than (+N), less than (-N),
or exactly N days in the past
-mmin MINS mtime is greater than (+N), less than (-N),
or exactly N minutes in the past
-newer FILE mtime is more recent than FILE's
-inum N File has inode number N
-user NAME/ID File is owned by given user
-group NAME/ID File is owned by given group
-size N[bck] File size is N (c:bytes,k:kbytes,b:512 bytes(def.))
+/-N: file size is bigger/smaller than N
-links N Number of links is greater than (+N), less than (-N),
or exactly N
-prune If current file is directory, don't descend into it
If none of the following actions is specified, -print is assumed
-print Print file name
-print0 Print file name, NUL terminated
-exec CMD ARG ; Run CMD with all instances of {} replaced by
file name. Fails if CMD exits with nonzero
Não sei por que você está usando o Busybox find , mas tente usar /usr/bin/find . Ou -exec rm {} + (sem suporte para -execdir ) ou a solução xargs sugerida por Rinzwind.