Getting Errors when using Photos to Build a Slideshow

I am trying to create a slideshow. My Mac is running Catalina 10.15.7. (I'm not able to update to newer OS versions due to storage capacity limits.--yes, I tried cleaning up/deleting unnecessary files, it still doesn't give me enough memory to update the OS.)

I have 80+ photos that I want use to to make a slideshow--without pasting each photo on an individual slide in Keynote (or similar program). I thought I had the solution by using Photos for Mac. It was working well earlier today, but I think the issue came on when the # of photos in the album grew (and I have more to add still).

I am getting an error message that says "there has been an error while downloading one or more photos for the slideshow, please try again later." A second message pops up saying "Internet connection required". However, my Internet connection is working just fine.

Any suggestions as to what I can do to get Slideshow working again, or can you suggest an app from the App store that can easily/quickly generate a slideshow of 100+ photos on an external drive?