Disk Utility First Aid errors after Time Machine disk created.

Using Big Sur I formatted a Sandisk Extreme Portable SDD 1TB as APFS using Disk Utility. It created three containers (disks). Ran the First Aid option on each and got no errors. Then in Time Machine I selected the SDD and the option Encryption. Ran First Aid after the Time Machine backup. Got Errors. The SDD seems fine; I'm able to read the files on it. But concern that there is still an issue and don't want to lose my backup files. I repeated the process again of formatting of the SDD and creating is as a Time Machine backup and got the same First Aid errors. Does anyone know what the issue might be?

Below I show the text from First Aid.

Running First Aid on “SanDisk Extreme 55AE Media” (disk5)

Checking prerequisites

Checking the partition list

Checking the partition map size

Checking for an EFI system partition

Checking the EFI system partition’s size

Checking the EFI system partition’s file system

Checking the EFI system partition’s folder content

Checking all HFS data partition loader spaces

Checking booter partitions

Checking Core Storage Physical Volume partitions

The partition map appears to be OK

Operation successful.

Running First Aid on “Container disk6”

Repairing storage system

Performing fsck_apfs -y -x /dev/disk5s2

error: container /dev/rdisk6 is mounted.

Storage system check exit code is 65.

Storage system verify or repair failed. : (-69716)

Operation failed…

Running First Aid on “Sandisk Backup_2” (disk6s1)

Repairing file system.

Volume was successfully unmounted.

Performing fsck_apfs -y -x /dev/rdisk6s1

error: container /dev/rdisk6 is mounted.

File system check exit code is 65.

Restoring the original state found as mounted.

File system verify or repair failed. : (-69845)

Operation failed…