How to Open a .PST file from the new outlook. File menu disapeared

I have old backed up emails in .pst format but I cant seem to be able to read them anymore in outlook on a new computer. I had saved the mails from old computer as .pst files and I see them in the outlook express but if I try to open them or even click on the backup folder, I get a error message:

"The set of folders cannot be opened. The server is not available. Contact your administrator if this continues "

What is the meaning of this message ? How do I get past it and read my old mails saved as .pst files from old computer ?

I use an Archive.pst file stored on a removable disk which I carry or store in a safe place when away from my laptop. I have "Quick Steps" programmed to move mail in one click to folders in the PST such as Family History, Financial Archive, Archived bills, etc. Does anyone know how I can replicate these functions when they finally force me off my windows version of outlook. Or at least retrieve the last 35 years of history. And yes, I have had email since 1979.

SaaS is where the money is, hence, Microsoft crapifying Outlook so that the web app doesn't look as simplistic.

I am somewhat back to normal. A while ago gmail said it would stop working with less secure apps like Outlook pre 2013 using POP. We upgraded to Outlook 2021 which used IMAP. I map using a ost format rather then POP. All well and good but or file history was in a pst file so I lost a lot of old documents. Since I changed to Outlook 2021 I could go back to POP. The delema was I need to choose, recent stuff on ost or old files (but importand files ) on pst.

I was looking for Microsoft help in transisioning but nada. So I decided the pst was more important then the ost.

I still have the ost file (which you cannot access outside Outlook, hoping someday Microsoft would accomidate old customers. Guess not.

Ed