You don't need AS keyword after the table. Also you have use AS in the end of the query to 'name' the name it, that's not correct in SQL syntax if this is the whole statement.
Try something like
SELECT top 100
a.[CUST_NUM],
a.[BILLED_AMOUNT_202008],
a.[BILLED_AMOUNT_202007],
b.[PROD_FAMILY_GRP],
b.[PROD_FAMILY],
b.[PROD_LINE]
FROM [MOBILE_PLANNING].[dbo].[C_prod_grp_202108] b
LEFT JOIN [MOBILE_PLANNING].[dbo].[C_mob_br_specialist_details_202012] a
ON a.[GL_ACCOUNT_CODE] = b.[SEF_CD]