Hey there, sakile180!
Thanks so much for using Apple Support Communities.
Being able to export your project is super important. We don't want your hard work and creativity to go to waste. Let's see how we can help!
If you need to free up space on your iPhone, here is how you can do that: How to check the storage on your iPhone, iPad, and iPod touch. It's also always a really good idea to make sure that you're checking your available storage on the device and not in iCloud. These can be confusing but it's an important distinction to make since iCloud storage would not pertain to files you're exporting from iMovie. Here's how: What's the difference between device storage and iCloud storage?
You can also change your export settings in iMovie which can lower the overall file size. To do that, tap your project from the main iMovie window, tap the Share button and then 'Options' at the top. You can then adjust the settings here which will affect the final exported project size.
We hope that this information is helpful.
​Have a great day!