First check that your theme structure looks like this under,
app/design/frontend/<vendor>/
├── <theme>/
│ ├── etc/
│ │ ├── view.xml
│ ├── web/
│ │ ├── images
│ │ │ ├── logo.svg
│ ├── registration.php
│ ├── theme.xml
│ ├── composer.json
Check all the files properly.
And delete var/generation
files and then refresh any admin page having added the theme files to the files system, your theme gets registered and added to the database
And also delete any entry made in database of your theme ,if any.
Or you can send your code so i can check whats's wrong.
Hope this helps you..