Thumbnail size not available through WordPress uploads

I am currently designing a website and I have a problem when trying to use the thumbnails in the Ultimate Product Catalogue.

If the thumbnails are activated in the backend the plugin is trying to include the picture with the ending “-300×300”. (In case I understand this correctly, the ending is added in Shortcode.php in line 653.) When I now go to the frontend no picture is visible.

I logged onto the the server via FTP and found that my pictures have been uploaded in several sizes, but not 300×300.
The sizes available are: 150×150, 222×300, 270×250, 570×350, 758×1024, 758×350, 758×500 and the original file without an ending. My thumbnails keep their proportions and are uploaded with a maximum width or height of 300px. Therefore only one of the two dimensions has 300px and the other one is smaller and might differ from picture to picture. I just don’t have the size 300×300 the plugin is demanding.

Is there any possibility to use the thumbnail feature when I don’t have the a picture in the size of 300×300 available? Could the plug dynamically check which size is available or so? Using (medium) thumbnails works everywhere else I’m using it on the website. Does WordPress have a possibility to request the URL for a medium sized picture? I would really appreciate a more dynamic way of getting the thumbnail URL.

https://wordpress.org/plugins/ultimate-product-catalogue/