builtin [ , -bash: [: missing `]'

The [ version of the command requires ] as the mandatory last parameter (so it must be preceded by a space). It’s just a formal, syntactic thing to force users to close the bracketed “block”, so commands look this way:

if [ $1 -eq 2 ]; then

instead of

if [ $1 -eq 2; then

See help [:

$ help [
[: [ arg... ]
    Evaluate conditional expression.

    This is a synonym for the "test" builtin, but the last argument must
    be a literal `]', to match the opening `['.

Notes

• The test version does not require nor accept the final ].

• You don’t need the builtin there. Builtins take priority over external commands, so just [ would execute the builtin. The builtin command is more useful when you have a function or alias masking [.