How to fix Error 555 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 555 — “User-Defined Functions Are Not Yet Enabled”

Why This Error Happens

This error typically occurs when a database is running under a compatibility level that predates support for user-defined functions, often after migrating or restoring a database from a much older SQL Server instance. It can also surface when the database engine is operating in a restricted or legacy parsing mode due to misconfigured compatibility settings. In some cases, corrupted system catalog views or an incomplete upgrade process on Windows Server 2025 can leave SQL Server 2025 unable to recognize UDF-related T-SQL syntax.

Step-by-Step Resolution Guide

Step 1: Check and Update Database Compatibility Level

  • Open SQL Server Management Studio (SSMS) and connect to the instance.
  • Run the following query to check the current compatibility level:
  SELECT name, compatibility_level FROM sys.databases WHERE name = 'YourDatabaseName';
  • If the compatibility level is below 130 (SQL Server 2016), update it:
  ALTER DATABASE YourDatabaseName SET COMPATIBILITY_LEVEL = 170;

Step 2: Verify SQL Server Edition and Feature Availability

  • Confirm you are running an edition of SQL Server 2025 that supports user-defined functions (Standard, Enterprise, or Developer editions).
  • Check installed features via:
  Start > SQL Server 2025 Installation Center > Maintenance > Edition Upgrade
  • Ensure no feature-limiting flags were applied during a scripted or unattended install on Windows Server 2025.

Step 3: Repair or Rebuild System Catalog Views

  • Run DBCC CHECKDB to rule out catalog corruption:
  DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS;
  • If corruption is found, restore from a known-good backup or run repair options as recommended by the DBCC output.
  • Restart the SQL Server service via Services.msc or:
  net stop MSSQLSERVER && net start MSSQLSERVER

Step 4: Reapply the Latest Cumulative Update

  • Download and install the latest Cumulative Update (CU) for SQL Server 2025 from the Microsoft Update Catalog.
  • Run the installer with elevated permissions on Windows Server 2025 / Windows 11.
  • Reboot the server after installation to ensure all binaries and system objects are refreshed.

Alternative or Verification Steps

  • After applying fixes, test by creating a simple scalar function (CREATE FUNCTION dbo.TestFn() RETURNS INT AS BEGIN RETURN 1 END;) to confirm UDFs execute without Error 555.
  • If the issue persists, consider restoring the database into a fresh SQL Server 2025 instance to rule out lingering compatibility artifacts from the original source server.