Here's some code that exports all the domains to Excel files. Also, you are getting the error when trying to export to DBF because the word "description" is 11 characters long.
''' Export all coded value domains in a geodatabase to Excel files in the same directory '''
import os, sys
import arcpy
## Ideal when testing so you don't keep getting errors
arcpy.env.overwriteOutput = True
## Specify the File Geodatabase workspace
gdb = sys.argv[0]
## Get a list of domains
desc = arcpy.Describe(gdb)
domains = desc.domains
## Loop over the list of domains
for domain in domains:
## Create an object that represents the name of the Excel file to be created
table_name = domain + '.xls'
## Let the user know what is happening
print('Exporting {0} domain to table {1}').format(domain, table_name))
## Create an object that represents the full path of the Excel file to be created
table_full_path = os.path.join(os.path.dirname(gdb), table_name)
## Create an in memory object for the DBF to temporarily store the domains (since this is the default file type)
in_memory_dbf = "in_memory" + "\\" + domain + ".dbf"
## Export the domain to the temporary in memory table
## NOTE: Cannot use "description," that is longer than 10 characters
arcpy.DomainToTable_management(gdb, domain, in_memory_dbf, 'field', 'desc', '#')
## Convert the in memory table to an Excel stored on disc
arcpy.TableToExcel_conversion(in_memory_dbf, table_full_path)
## Clear the memory so ArcGIS doesn't pitch a fit
arcpy.Delete_management("in_memory")
EDIT: fixed print format (line 20)
