I tried creating a custom rectangular weldment profile and constrained it fully, it is saved in the library but when using in weldment part it does not show up in the list and also no message is shown
Weldment profiles are slightly different from regular sketches.
How to create a new weldment profile:
open my computer ->local hard drive-> program files-> solidworks install folder-> data-> weldment profiles-> ANSI inch-> and then one of the profiles folder.
When you have opened the folder, copy and past the same file so you have the original file as "profile.sldprt" and the copy file as "copyofprofile.sldprt".
Drag and drop(do not use open) the copyofprofile.sldprt into solidworks, change the dimensions and save it with a new name.
After saving it, it should be ready for use :).
Let me know if you still have trouble!