Again, yes, you have deleted all the local snapshots. But you clearly have an external Time Machine drive. And Time Machine snapshots (local or remote) place certain requirements on the source drive. From
BACKUP STRUCTURE Throughout this manual, specific language is used to describe particular "realms" associated with Time Machine backups. It is important to under- stand this terminology to make effective use of tmutil and its manual. backup source A volume currently being backed up by Time Machine. backup disk The HFS+ volume that contains Time Machine backups. backup destination In the case of a local destination, a synonym for backup disk. For network destinations, this is the AFP or SMB share on which the backup disk image resides. backup disk image (or backup image) A sparsebundle that, when mounted, is the backing store for a volume that is a backup disk. backup store The top-level "Backups.backupdb" directory at the root of a backup disk. E.g., /Volumes/Chronoton/Backups.backupdb machine directory A directory inside a backup store that contains all the backups for a particular computer. For local destinations, a backup store can contain multiple machine directories, all for separate com- puters. E.g., /Volumes/Chronoton/Backups.backupdb/thermopylae
Remember the error?
E.g., (perhaps caused by APFS Snapshot usage by Time Machine)?
Read this one:
snapshot A directory inside a machine directory that represents a single initial or incremental backup of one computer. The word "snap- shot", in most contexts, is a generic term and is not to be con- fused with a "local Time Machine snapshot", which is simply a snapshot stored locally on the computer. E.g., /Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456
snapshot volume A directory inside a snapshot that represents a single initial or incremental backup of one backup source. E.g., /Volumes/Chronoton/Backups.backupdb/thermopy- lae/2011-07-03-123456/Mac HD VERBS Each verb is listed with its description and individual arguments. destinationinfo [-X] Print information about destinations currently configured for use with Time Machine. For each backup destination, the following information may be displayed: with Time Machine. For each backup destination, the following information may be displayed: Name The volume label as shown in Finder. Kind Whether the destination is locally attached storage or a network device. URL In the case of a network destination, the URL used for Time Machine configuration. Mount Point If the volume is currently mounted, the path in the file system at which it was mounted. ID The unique identifier for the destination. When more than one destination is configured, the most recent backup destination will be marked with the > indicator. When the -X option is provided, output will be printed in XML property list format. removedestination identifier Remove the destination with the specified unique identifier from the Time Machine configuration. Requires root privileges. To obtain the unique identifier for a destination, see destinationinfo.
A. The error:
Error: -69521: Your APFS Container resize request is below the APFS-system-imposed minimal container size (perhaps caused by APFS Snapshot usage by Time Machine)
B. Your response:
I have removed all local time machine snapshots but I am still getting the same output.
C. The documentation:
The word "snapshot", in most contexts, is a generic term and is not to be confused with a "local Time Machine snapshot".
Yes, you have deleted all your local snapshots. But you also have another backup drive. In the example you linked, there was no such drive.
You need to remove all Time Machine backups stores, disable all destinations, etc. You can reassociate your backups later. But following this, you should be fine; worst case, decrypt your drive too (APFS encryption is complicated and has a lot of overhead due to the drive-key/user-key system).