Why AE 2023 rejects AE 2026 projects
The .aep file contains version signatures and project chunks written by AE 2026. Older AE 2023 builds may not understand those structures, so the project is blocked before normal editing starts.
Recommended AE 2026 to 2023 workflow
Keep the original file, create a downgraded copy for AE 2023, then open that copy in the target version and inspect comps, layers, footage paths, expressions and effects.
Compatibility limits
A version downgrade can adjust project structure, but it cannot add AE 2026-only features to AE 2023. New effects, expressions, properties or plugins may require manual replacement.