Unable to submit answer for Task 15: Let's draw a Diamond!

I have met a problem with “Variables! Safe place to store your temporary data!” Step 1 Task 15.
I have submitted my answer three times but it shows me that I am still unable to save the task. After three times of submissions, my submit button is disabled now, and I’m also cannot proceed to solve the following tasks.

I log in to Cubie in a new tab and open the same step same task again. Then, my answer is able to be submitted.

The captures are attached below.
First try:

Second try in a new tab: