when creating RAID1 in RHEL8 getting mdadm failed device or recourse busy

Can we discuss "How to ask meaningful questions?",?
Because that question was not asked in a way that can lead to any meaningful answer.

We need to know what you were doing. The exact command line might help.
We need to know what the devices and the existing partitions and file systems look like to compare with your command line.
If you were following instructions, it will help if you direct us to your instructions and verify exactly which steps you completed and at what point you generated the error.

YOU, however, may not need to tell us that if you have a properly registered instance of RHEL! RHEL has excellent support pages and a fine service team, and you should be able to check those resources. (Unless you are running unlicensed, in which case no one can help you directly.)