Android App Email Verification Issue Fix

Requirement: Immediate: Process: com.samphion, PID: 5055
java.lang.IllegalArgumentException: Unable to create call adapter for io.reactivex.Observable<com.samphion.api.repo.ApiResponse<com.samphion.api.pojo.token.TokenResponse>>
for method RestApi.sendVerificationEmail
at retrofit2.Utils.methodError(Utils.java:56)

Resolution Timeline : 1-2 hours ( have earlier rolled out such collaborative tasks with same requirement within 1-2 hour )

I'm seeking an experienced Android developer to resolve an email verification issue in my mobile app. Currently, verification emails are not being sent, and I am using an google SMTP nodemailer. Almost all configurations and code have been deployed, only final roll-out is remaining

I'm seeking a developer to implement user registration for email signup/login authentication.

Requirements:
- User registration form with email address and password fields
- Implementation of email verification during registration

Ideal Skills & Experience:
- Proficiency in Android, Java, Node.js, and MongoDB
- Experience with user authentication and email verification processes
- Strong backend development skills to handle API requests and responses

All code pertaining to frontend xml files having send, receive parameters ; Java code pertaining to send and receive parameters have been created. Code pertaining to backend - email generator, email send, verify, router, auth, database connection has been created

Node.js files have been created
- SendEmail
-VerifyEmail
- Routes
- user Model
- otp generator

Front end
- xml codes
- Java code
for login, send and receive mail has been created

1. Workflow:: "Android Mobile app has been integrated with Login/Signup process having email id and password for signing in the platform. User receives verification over email id and
that is utilized to verify credentials. New user is navigated to SignUpStep1Activity Screen and existing user is navigated to Main Activity Screen"

2. Objective : To achieve friction-free login/signup process using email id and verification

3. Tech Stack: Node.Js, MongoDB, Java ( Android ), Android Studio, Linux

4. Current Stage: Current build is working well without any error or glitches. Build process has been successfully accomplished. However the login process is not resolving at "SendVerificationEmail" method

5. Accomplished Zone: "All Front End and Backend files pertaining to requests and response including of xml, java, .js, .env have been coded. Sample code is also getting generated to test email id.
Build is error free at front end ( Android Studio ) and Backend Code is also pushed to live which is initiated without any glitches"

Preliminary Workflow Parameters
A. Access will be provided to Client Device and any request for code or git will not be provided
B. Milestones based payments will be provided.
C. Working on two codes or two assignments of two clients will lead to immediate termination
D. It is requested to remove all code, credentials and files in your device post completion of tenure and avoid sharing it with anyone

Requirement: Immediate
Resolution Timeline : 1-2 hours ( have earlier rolled out such collaborative tasks with same requirement within 1-2 hour )

Please provide a detailed plan on how you would approach this issue.