Hi,
You can check out this related topic;
https://www.wpbeginner.com/wp-tutorials/how-to-fix-503-service-unavailable-error-in-wordpress/
Here’s what I found regarding the 503 error:
The 503 Service Unavailable error is a server-side error, meaning the problem is usually with the web site’s server. … Even though the 503 Service Unavailable error means that there’s an error on another computer, the issue is probably only temporary.
Also, just be sure that you increased the upload limit at least to 128mb. If you still have the issue, you need to contact your hosting provider about increasing the allocated memory size for your site.
When I checked your website, I saw that you already imported the demo data as well.
Have a nice day.