Fix Google Indexing Issues

I have a small website with roughly 30 live pages, yet Google Search Console is only indexing 11 of them. The coverage report flags a few pages with errors, but I need a deeper diagnosis to understand exactly why the remaining URLs are being excluded and, more importantly, to get them indexed... (Budget: $8 - $15 AUD, Jobs: Analytics, Content Management System (CMS), Content Strategy, Data Analysis, Search Engine Marketing, SEO, Web Development, Website Audit, Website Management, Website Optimization)