Hi,
first step to solving this issue is to get a good error message, please activate the WordPress Debug mode: https://wordpress.org/documentation/article/debugging-in-wordpress/
With the error message we can figure out what is causing the problem. (Please post the error)
If we don’t get a good error message a great way to start debugging your problem would be to deactivate all your plugins and see if the problem persists, if it doesn’t reactivate them one by one and you will see which plugin causes the problem.
Deactivating a plugin can be done by renaming it’s folder in /wp-content/plugins/ and reactivating can be done by renaming it back to its original name.
What exactly happens when you try to upload a file? Is an error message displayed and if so, which one?
Please also look under Tools > Site Health to see if any anomalies are mentioned there.
