Ah; I see - the Project Panel icon view for srt in PR 2021 is the Filename and Pr symbol.
In PR 2021, you cannot use "Modify" on a caption file.
If you are importing the srt and then applying it to a caption track, do it the other way around: go to the sequence you want to use, go to the Captions workspace, or just open the Text Panel, and use the "Import captions from file" button. There are no settings that you can access, and I have assumed that the caption track is set to the sequence. I do not know this for a fact.
Your file name appears to be closed captions. I think it is more likely that you are simply running into the character limits for closed captions - 32/42 per line.