I cannot open an old iPhoto library in my new Mac. what can I do? thank you

+1 on the importing of the Masters or Originals folder. This is because if the library was created with iPhoto 7 or earlier it will need too be modified by the iPhoto Library Upgrader app and that app only runs on macOS Mojave or earlier.

If the OP can get his library to a Mac running Mojave or earlier he can run the upgrader on it and then migrate it to Photos. The upgrader can be found at the Apple Downloads page.

You need to convert it to a Photos library. Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will create a new Photos library from your iPhoto media. Your iPhoto Library will not be changed, though it will be renamed.

If you run out of options, you can control-click on the library and choose "Open Package Contents" navigate to the Masters or Originals folder and you can access your images there and import them into Photos (File->Import to Library) or handle them as you like.

Do you have the old Mac still? You need to run the iPhoto Library updater on it. It is 32 bit code, so you will have to run it on an OS before Catalina.