How can I add a contact form to the sidebar of Tumblr?

How can I fix this, and also how can I add the contact form to the sidebar instead of just as a post?

I would like to recommend you to check the following article to get the idea about the Tumblr sidebar editing.

How to Customize Your Tumblr Blog's Sidebar

You will need to edit your template HTML to inject the form code.

Here is an example where the form code is added after the buttons section in sidebar.

I used the following code:

<div id="jotform">
<Form Script Code Here>
</div>

You can try to edit your sidebar in the same way.

Alternatively, it is possible to create a separate form page, and link it to the sidebar. See the following thread:

Contact form in Tumblr Sidebar

If you need any further assistance, please let us know.