As @Gary advised normally if you are clicking ok it generally means you have “either multiple accounts setup”, or “have the account setup wrong” in some way, or “have some problem with the mailbox authorisation on the server end”.
@skybat advised POP options you can try in that video example (or that POP thread at the top), but “no guarantee that will continue to work” as per Google changes. You would also normally need to remove and re-add your POP account if you try that option in the video or thread.
Also if you do remove your POP account and re-add it to try, create a folder (first) down in “Local folders” at the bottom left called eg: Old Mail and then drag all your exiting POP mail under that. Then once you are confident you have all your email down in Local folders, you can then remove your old POP account. You can then re-add your new account and then drag all your old email & folders to the new account. Good luck.
eM Client as a company also does not support that now with Google Gmail workspace accounts.
As @Gary and I advised "as per eM Client announcement earlier in the year with Google’s changes, you are now meant to be changing to IMAP via the “automatic email setup” with the OAuth token, and dragging / moving all your POP emails to Gmail IMAP account folders.