Yes the identifier is algebra but there is no third parameter so the underscore should not be here, just something like
[ [ 1:algebra ] ]
Note that you should not put any space between the brackets but I added spaces here so that they are displayed in the forum
See the help here https://docs.moodle.org/36/en/Algebra_question_type#Using_Algebra_in_combined_questions
And when you create a new combined question in the question text there is a sample of the pattern you need to use for each subquestion type that is combinable to help you to find the right syntax.
As this feature is new, please tell me if you manage to make it work or not.
If you still get the same error message when you use the right syntax, please ask your sys manager to verify that he installed the right version of the algebra question type: there should be a subfolder named combinable in the question/type/algebra folder.
I am also interested in reports of Moodle users if they try to use algebra question in quizzes on the mobile app as I was not able to test this myself.