1. Create an Assignment
2. Fill in Assignment name "Test"
3. go to Completion tracking
4. select require grade 
   1. Student must receive a grade to complete this activity and

1. 1. Student must receive a passing grade to complete this activity

You will get this: Exception - grade_floatval(): Argument #1 ($number) must be of type ?float, string given, called in [dirroot]/course/moodleform_mod.php on line 496

Stack trace:

• line 1595 of /lib/gradelib.php: TypeError thrown
• line 496 of /course/moodleform_mod.php: call to grade_floatval()
• line 244 of /mod/assign/mod_form.php: call to moodleform_mod->validation()
• line 653 of /lib/formslib.php: call to mod_assign_mod_form->validation()
• line 589 of /lib/formslib.php: call to moodleform->validate_defined_fields()
• line 699 of /lib/formslib.php: call to moodleform->is_validated()
• line 1352 of /course/moodleform_mod.php: call to moodleform->get_data()
• line 166 of /course/modedit.php: call to moodleform_mod->get_data()

The problem may be that the passing grade is still empty. If one makes comparable steps with an existing task, an error message is immediately displayed under "Student must receive a passing grade to complete this activity".

Notice: I have not tested the other modules with the passing grade option.

Also tested on https://qa.moodledemo.net/