When it happened to me I shut down the new version of Vegas, did an image sequence render with an earlier version, and after that the new one worked. Voodoo, I know, but it worked...
@Chris-Brown Search the forum and you will find several useful threads. Two questions - are you Win 10 or Win 11 and what is your VP version and build number?
In the 'Render As' dialog box, there is a Filters option near top left. If it says 'Filters On', then click to drop down the filters options and clear any that are selected. Several of those options will not show Image Sequence.
I have a new computer, so I only have like 5 projects so far, but the older ones did have a lot more options for me to choose from in the render templates. But still not Image sequence.
Do you have Happy Otter Scripts? There appears to be two ways to do this in HOS---direct access to the Vegas command (maybe it will show up there? I don't know...) or as an ffmpeg command. There may be other options there...