Hi Muhammad,

Causes of the 'Record type unavailable' error when converting a Lead
1. The Organization has Person Accounts enabled and the affected User's profile does not have access to the Person Account and/or Business Account record types.

Salesforce Classic
Click Setup.
Under Manage User, click on Profiles.
Select the Profile of the affected user.
Under Record Type Settings Section, click on Edit.
Add the Person Account Record Type.
Click Save.
Lightning Experience
Click the gear icon.
Click Setup.
Under Users, click on Profiles.
Select the Profile of the affected user.
Under Record Type Settings Section, click on Edit.
Add the Person Account Record Type.
Click Save.
**Steps for Orgs that have Enhanced Profile User Interface is enabled in Lightning Experience

Click the gear icon.
Click Setup.
Under Users, click on Profiles.
Select the Profile of the affected user.
Select Object Settings
Select the Object
Click Edit
Ensure that that the box in the person accounts row is checked under assigned record type
Also ensure that it was a default record type.
Click Save.
2. Either the record owner or the User converting the record lack access to the appropriate record type.
3. The affected User's profile has the default record type set to '-Master-' for the Account, Contact, Opportunity and/or Task objects.
To resolve this error, check the points above.  Make sure that the record types are made available for the profile, and that the default record type is set appropriately for the objects listed above.

Please mark as Best Answer so that it can help others in the future.

Regards,

Priya Ranjan