Below are some examples of the sign-up shortcode . One thing to note is lists need to be public for someone to subscribe to them. To check that a list is public you can hit the edit link below the list name on the subscribers page.
Full ShortCode with all options:
[sendpress-signup firstname_label='First Name' lastname_label='Last Name' email_label='E-Mail' listids='1' display_firstname='true' display_lastname='false' label_display='false' desc= '' label_width=100 thank_you='Thank you for subscribing!' button_text='Submit']
To use multiple lists in the short code, separate each list id with a comma and the shortcode will display each list with a checkbox next to it so the user can choose to subscribe to as many lists as they want to subscribe to.
Here’s an example of your shortcode with more lists added for comparison:
[sendpress-signup firstname_label='First Name' lastname_label='Last Name' email_label='E-Mail' listids='121,122,123,124' display_firstname='true' display_lastname='true' label_display='true' desc= '' thank_you='Thank you for subscribing CREC newsletter!' button_text='Submit']