rman error

I have the following script for 8i RMAN. I am getting errors to run backup.
run {
allocate channel ch1 type 'sbt_tape'
PARMS="BLKSIZE=262144,ENV=(CV_mmsApiVsn=2,CV_channelPar=ch1)"
TRACE 0;
setlimit channel ch1 maxopenfiles 8;
backup archivelog all delete input ;

backup current controlfile;

RELEASE CHANNEL ch1;
}

error

RMAN-03022: compiling command: XCHECK
RMAN-03026: error recovery releasing channel resources
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure during compilation of command
RMAN-03013: command type: XCHECK
RMAN-06091: no channel allocated for maintenance (of an appropriate type)