Export is still a work in progress. With the current Premiere Pro 14.9 Beta build 009, you could try this workaround until we get everyhting implemented.
1) Place SRT in Timeline, then add any audio or video to Timeline (currently required to enable Export Media, but we hope to address this soonish).
2) Go to Export Media and choose any Format (eg: H.264). Uncheck Export Video (to save time and unwanted video). Go to Captions tab and choose Export Options: Create Sidecar File, then choose File Format: Subrip Subtitle Format (.srt).
3) Click Export button.
This will produce an audio file (discard) and your desired SRT file on disk.