Try this IF( ISPICKVAL( [In_Form__Risk_Assessment__c].Risk_present_MH__c , "No" ) , "No risk" ,
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) < 8, "Low",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 8, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 9, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 10, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 12, "Medium",
IF( CASE([In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) > 12, "High", "" )))))))
IF( ISPICKVAL( [In_Form__Risk_Assessment__c].Risk_present_MH__c , "No" ) , "No risk" ,
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) < 8, "Low",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 8, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 9, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 10, "Medium",
IF( CASE( [In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) == 12, "Medium",
IF( CASE([In_Form__Risk_Assessment__c].Likelihood_mental_health__c , "Very likely (5)", 5, "Likely (4)", 4, "Possible (3)", 3, "Slight (2)", 2, "Very unlikely (1)", 1, 0) * CASE( [In_Form__Risk_Assessment__c].Severity_of_impact_mental_health__c , "Critical (5)", 5, "Major (4)", 4, "Significant (3)", 3, "Minor (2)", 2, "Insignificant (1)", 1, 0) > 12, "High", "" )))))))