Hi,
Hope you have tried searching forum before posting this kind of basic question. As per forum rules basic questions has to be searched first
However you being a new member , as a welcome note i am suggesting you how to resolve this
this might be because, your enterprise structure is not properly configured
Please check
1. Whether Plant was assigned to Company code
2. Check whether Sales organization/distribution channel and Plant assignment is done
after properly configuring this you have to enter Plant
1. customer master
2. Material master
3. Customer material Info record
If all the above are done then automatically system will determine plant or you can manually enter the Plant inn the sales order provided if you have maintain enterprise structure
check and revert
regards,
santosh