Can't export table structure with expdp, parameter full=X conflicts with tablespaces=X
What you posted works fine.
[oracle@o81 ~]$ expdp \'/ as sysdba\' tablespaces=users content=metadata_only

Export: Release 19.0.0.0.0 - Production on Sat Nov 14 12:48:17 2020
Version 19.9.0.0.0

Copyright (c) 1982, 2019, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Starting &quot;SYS&quot;.&quot;SYS_EXPORT_TABLESPACE_01&quot;:  &quot;/******** AS SYSDBA&quot; tablespaces=users content=metadata_only
Processing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
Processing object type TABLE_EXPORT/TABLE/TABLE
Master table &quot;SYS&quot;.&quot;SYS_EXPORT_TABLESPACE_01&quot; successfully loaded/unloaded
******************************************************************************
Dump file set for SYS.SYS_EXPORT_TABLESPACE_01 is:
  /u01/app/oracle/homes/OraDB19Home1/rdbms/log/expdat.dmp
Job &quot;SYS&quot;.&quot;SYS_EXPORT_TABLESPACE_01&quot; successfully completed at Sat Nov 14 12:48:43 2020 elapsed 0 00:00:26

[oracle@o81 ~]$ 

You get this error when the tablespace specirfied does not exist:
[oracle@o81 ~]$ expdp \'/ as sysdba\' tablespaces=users123456 content=metadata_only

Export: Release 19.0.0.0.0 - Production on Sat Nov 14 12:50:16 2020
Version 19.9.0.0.0

Copyright (c) 1982, 2019, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Starting &quot;SYS&quot;.&quot;SYS_EXPORT_TABLESPACE_01&quot;:  &quot;/******** AS SYSDBA&quot; tablespaces=users123456 content=metadata_only
ORA-31655: no data or metadata objects selected for job
Job &quot;SYS&quot;.&quot;SYS_EXPORT_TABLESPACE_01&quot; completed with 1 error(s) at Sat Nov 14 12:50:19 2020 elapsed 0 00:00:03

[oracle@o81 ~]$