A referenced class or method has compilation errors.

Problem Explanation

Salesforce validates all dependencies during deployment.

Root Cause(s)

1. Method signature changed

2. Deleted class reference

3. Namespace conflicts

Step-by-Step Solution

1. Compile all Apex classes in target org

2. Fix dependency errors first

3. Redeploy in correct order

Edge Cases & Variations

1. Managed packages lock dependencies

2. API version mismatches cause issues

Common Mistakes to Avoid

1. Partial deployments

2. Ignoring compile warnings