The error indicates the Dept table dept_code column is the deprecated text data type. This is most likely database design flaw since you probably don't need 2 billion characters for a dept_code value.
It would be best to fix the column type to match the MTD table dept_code column type but a temporary work-around is a substring of the value in the query:
select
a.cust_no,
a.dept_issue,
a.dept_code as dept_code,
b.name as dept_name
from [dbo].[Dept] as a
left join [dbo].[MTP] as b
on SUBSTRING(a.dept_code, 1, 6) = substring(b.dept_code,3,6);