Unable to open any of the demos (error CS0582: Attribute 'SerializeField' is not valid on this declaration type. It is only valid on 'field' declarations.)

[MOSMekawy]

I tried multiple unity editor versions but none of them work. Some crash and some just throw error.
My latest attempt was using this config/environment.

Environment:

• Windows 11
• Unity 6.4 (6000.4.0f1)
• SofaUnity-URP_v25.12_public_2025-03-20

Error:

• error CS0582: Attribute 'SerializeField' is not valid on this declaration type. It is only valid on 'field' declarations

Warnings:

• The GUID inside 'Assets/SofaUnity/Core.meta' cannot be extracted by the YAML Parser. Attempting to extract it via string matching instead. Please verify the file does not contain unexpected data.
• ImportFBX Warnings: Can't import normals, because mesh 'default' doesn't have any.
• Mesh 'default' has no normals. Recalculating normals.