how can I use american sign language with mac? Is there another program I need?

I don't think I understand your question. Who are you trying to communicate with using ASL? Deaf people I know use Facetime or Zoom. Do those options not work for you?

Or are you asking if there is a way to transcribe ASL to text (or English) speech automatically? No, that is not a Mac feature. From what I can tell from a quick internet search, it's not something that it seems many people are doing. I only found one:

https://slait.ai/

It doesn't seem to be Mac specific.

Here are is another resource page:

https://guides.lib.uh.edu/asli/apps

Can you just use iMessage? Or Facebook Messenger? Or whatever chat program you like?

You might consider Zoom. Zoom has a chat feature. So, you could do a Zoom call if you want to see their faces and use the chat window at the same time. Zoom also has a captioning feature so, what you say would be captioned for them.

I think the page I linked to above also has some resources for learning ASL.

Be sure to check out the captioning feature in Zoom, too.

I found ASL to be a lot of fun to study and all the people I know who are Deaf were always really excited that I made the effort and didn't even make fun of my accent too much.

How about tradition methods ... typing emails or messages.