If the laptop is now legitimately yours and you no longer need the organization's access, you can remove the work connection from Windows and then use your personal Microsoft account.
First, go to Settings > Accounts > Access work or school. Select the old work account/organization connection and choose Disconnect. Windows may warn that removing the connection will remove access to organizational resources. That is expected. If it asks you to authenticate with the old work account and you cannot, do not keep trying to use that account.
If the old work account is actually the Windows sign-in account rather than merely an organizational connection, the approach is different. You generally cannot remove the only administrator account while you are logged into it. Instead, create a new local administrator account or sign in with an existing personal administrator account first. Go to Settings > Accounts > Other users > Add account, choose I don't have this person's sign-in information, then Add a user without a Microsoft account. Create your personal/local account, change its account type to Administrator, and sign into that account.
Once you have confirmed that the new account works and has administrator privileges, return to Settings > Accounts > Other users, select the old work account, and choose Remove > Delete account and data. This removes that Windows profile from the PC. Be careful because deleting the profile also deletes files stored only in that user's Windows profile, such as files under C:\Users\<old-user>.
Afterward, you can connect your personal Microsoft account under Settings > Accounts > Your info and use that account for Windows. You can then purchase Microsoft 365 Personal and install Office using the same personal Microsoft account. You do not need your former employer's Microsoft 365 license to do this.
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hth
Marcin