could not read symbolic link error while executing pg_start_backup function in Postgresql DB

In order to take data folder backup in postgres 9.6 database executed the order of functions for backup, while executing *pg_start_backup* function produced *warning : could not read symbolic link for pg_tblspace Invalid argument*

test=# checkpoint; 
       CHECKPOINT

test=# select pg_switch_xlog();
 pg_switch_xlog
----------------
 5/DF000238
(1 row)
**
test=# select pg_start_backup(to_char(now(), 'yyyy-mm-dd hh24:mi:ss'));
WARNING:  could not read symbolic link "pg_tblspc/oth_data_tbs": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/str_index": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/tbs_temp_default": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/oth_indx_tbs": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/str_data_ids": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/tbs_default": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/top_indx_tbs": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/rpt_data_tbs": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/str_data_h_index": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/rpt_indx_tbs": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/str_data": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/str_data_h": Invalid argument
WARNING:  could not read symbolic link "pg_tblspc/top_data_tbs": Invalid argument
 pg_start_backup
-----------------
 5/E0000028
(1 row)