Hi there!
It may be that the sitter is already booked for another THS sit at that time and the site is preventing you from being able to send an invite. If this isn’t the case, it could be a glitch, particularly if you’re using the app on your phone (it can be a bit buggy).
2 quick things to know about THS:
1: Never rely on sitter’s calendars - they are usually incorrect because they are essentially unusable for sitters (it’s tricky for sitters to input dates that they’re unavailable besides being booked for another THS sit, and the calendars are often misleading & confusing).
2: You’re welcome to send invites if you like, but just know that invites are rarely successful, as the chances that a random sitter is available and interested in your particular sit are slim. The best way to secure a sitter is to have a really attractive listing that gains applicants, as these are the sitters that are available, searching for sits in your area, and interested in your sit. There are plenty of discussions about this on the forum already, if you use the search bar to find topics - for example, this recent thread: No active sitters
If you have low applicants so far and are seeking advice, here’s a blog I inspired for THS about mistakes to avoid when creating your listing. Hope that helps! 