Please make sure that Groove Agent 5 is up to date. Click the Groove Agent 5 logo to the top left to check which version you have installed. The current version is 5.1.11.
Also check if the Steinberg Download Assistant is offering any VST Content updates. These will show up above “My product downloads” if any are available.
Then follow these instructions, replacing “HALion Sonic SE_64” with “Groove Agent_64”:
https://helpcenter.steinberg.de/hc/en-us/articles/115001720190-FLUX-Wavetable-Synth-missing-in-HALion-Sonic-SE-3