This is common issue among react native developer .
Reasons of this

1. Node Package mismatch (95% of the time )
2. .gitignore file not available

SOlution
1.Delete node_modules , Delete package_lock.json and then do npm install on fresh package.json ( THis will fix your problem most of the time)

If the issue is not fixed

1. Create new project
2. Add dependencies(only) from your package.json and then do npm install
3. Run the app

4. Try building again this will work for most of cases

5. for git error just initialize git using "git init"