form-action.php

If the user might not be logged in, you also need to hook “admin_post_nopriv_acr_addRoom”.

The form tag’s action attribute should be a full, absolute URL to admin-post.php. Relative URLs will fail under certain conditions.

If you haven’t, define WP_DEBUG as true in wp-config.php

None of these may solve your problem. Find out if your callback is even called by error logging a message. There’s not much to admin-post.php. It gets the “action” value and fires the related action. You can temporarily place debug code in the file to verify all is working as it should. If your callback is called, it apparently is working. Then do a detailed debug of your callback, verifying every variable and conditional is working as it should.

You can use the Query Monitor plugin to verify the insert query’s SQL is as it should be.

@bcworkz thank you for reply. I think that the form is working & action is called. When I get redirected to blank post-admin.php I can see on Inspection screen, tab Network, that I have the data of my form.
I also realized, that I’m missing redirect. I added redirect, but somehow I’m still landing on post-admin.php blank page. I have also tried with action attribute as full, even though I have WP_DEBUG as true, but without sucess.

<?php
//Add Calendar to DB
function acr_addRoom() {
     if ( isset($_POST["name"]) && isset($_POST["maxUnits"]) && isset($_POST["partlyBooked"]) > 0){
     
	 //get values from form
	 $name    = sanitize_text_field($_POST["name"]);
     $maxUnits   = intval($_POST["maxUnits"]);
     $partlyBooked = intval($_POST['partlyBooked']);

	//inert into DB
	global $wpdb;
	$acr_rooms_table = $wpdb->prefix.'acr_rooms';
	$wpdb->insert(
            $acr_rooms_table,
            array(
                'name'    => $name,
                'maxUnits'   => $maxUnits,
                'partlyBooked' => $partlyBooked
            ));
	}
wp_redirect(  admin_url("admin.php?page=availability_calendar_rental_overview&setting=roomAdded"));
		exit;
}
add_action( 'admin_post_acr_addRoom', 'acr_addRoom' );
?>

<div class="wrap">
 <h1>Add Room Info</h1>  
  <form method="post" action="admin-post.php">
  <input type="hidden" name="action" value="acr_addRoom" />
	<table class="form-table">
	  <tr>
		<td><label for="name">Name</label></td>
		<td align="left"><input name="name" placeholder="Room Type A" id="name" type="text" class="regular-text code" required />
		</td>
	</tr>
	<tr>
		<td><label for="maxUnits">Number of rooms</label></td>
		<td align="left"><input placeholder="10" name="maxUnits" id="maxUnits" type="number" class="regular-text code" min="1" required />
		</td>
	</tr>
	<tr>
		<td><label for="partlyBooked">Threshold for "partly booked"</label></td>
		<td align="left"><input placeholder="1" name="partlyBooked" id="partlyBooked" type="number" class="regular-text code" min="1" required />
		</td>
	</tr>
	</table>
 <br />
 <input class="button button-primary" type="submit" value="Add Room" />
  </form>
</div>

Where are you actually placing your code? The action callback and add_action() call do not belong in the same place as your form HTML. If they are together as they are here, the PHP portion may not be evaluated when admin-post.php is requested.

Yes, you are right. This was an issue. I added function to external file & everything works.
Thanks again!