Class \Vendor\ModuleName\Api\TestInterface does not exist
During development move things from local-src + symlink to app/code/{vendor}/{modulename} so that namespace and folder structure could match.
If you are using interfaces in your module then this is also true that you need to use full path reference in PHPdoc instead of use import and reference.
Both worked for me
Specified request cannot be processed
rest/V1/test/ – V in version needs to be capital letter (also in webapi.xml).
And just useful tip that you can extract interface out of existing class.