How to Resolve Not Finding Default Record Type When Converting Leads

1.  Go to Setup (the gear icon in the top right, then select Setup).
2. In the Quick Find box, type 'Profiles' and select it.
3. Select the profile of the user experiencing the issue.
4. Under 'Apps', 'Object Settings' (or 'Object Permissions' in some older interfaces), find 'Leads' and click 'Edit'.
5. Ensure that the 'Default Record Type' for Leads is set to an active record type.
6. Verify that the user's profile has access to the desired record type.
7. Click 'Save'.

(Optional) Repeat these steps for any other profiles that need access to this default record type.
(Optional) Check the 'Lead Settings' in Setup to ensure that lead conversion settings are configured correctly.