This is the result of the Block Editor UI update that was rolled out without warning on 3rd February 2021. Prior to this update, you could manually enter multiple categories (or multiple tags) separated by commas on anything other than Squarespace 7.1 Store content. Unfortunately, the new Block Editor has a dropdown control instead of a textbox and it is now only possible to select one category (or tag).
Squarespace overcame many of the new UI limitations by issuing an update on 14 February, but this particular issue was not resolved in this update. I agree with @digitalfreelancer that this is extremely irritating and I hope this is being fixed now. You can voice your concerns to the Product team by logging a ticket here.
For information, the Summary Block Editor UI is shown below, before and after the update:
Hi @Margiada56 you're talking about the category and tag filters you can apply to summary blocks. Previously it was an open text field so you could comma separate multiple tags/categories, but now it's a dropdown so you can only select one.
I agree that this is a severe limitation in the latest update and hope that Squarespace see sense that it is really useful to be able to filter by multiple categories/tags.