Switched to branch version-11 and updated to latest version 11 .64 successfully.
After that tried to switch to branch version-12 encountered missing doctype error “Milestone Tracker” and someone mentioned in another thread just create back the entry under tabDocType and it worked.
Second problem, not sure what is the cause:
It occured during:
Executing frappe.patches.v12_0.move_timeline_links_to_dynamic_links in site1.local
pymysql.err.IntegrityError: (1062, “Duplicate entry ‘1000000001’ for key ‘PRIMARY’”)
Any idea how to get this fixed? Switching back to version-11 now.