How can I start a library together with Pure Data? I tried the path File --> Preferences --> Startup, but when I start Pd the message "Pd can't load library" appears

@blurryface21 There are 2 types of library.
Most have a separate binary for each object, so Pd can find them easily by name, and you just have to add the path to File - Preferences - Path ...... so that Pd knows where to look when you create an object.

Some have a single binary that contains all of the objects, and obviously Pd cannot then find the objects by name.
So for those libraries the single binary must be loaded as Pd starts..... then Pd is aware of all the objects contained in the single binary.

To add a single binary library to "startup" the format is for example....... -lib Gem -lib iemnet

Using the File - Preferences - Startup box is exactly the same as setting a command line switch when starting Pd from a terminal or using a batch file....... https://puredata.info/docs/faq/commandline

If it is a single binary library then check the name....... Gem for example, not gem...... I don't think any others start with a capital.
Is it a single binary? or do you see multiple binaries with recognisable object names in the folder.
Did you add the path to the single binary in File - Preferences - Path... ?...... Pd still needs to know where to find it.......... Pd only looks in its standard paths otherwise (bin and extra).
Is it an old 32-bit library, and you are running 64-bit Pd.... or the other way around?
David.