We are trying to generate JWT access token. Code for the same is provided below:
 

ContentVersion cv = [SELECT VersionData FROM ContentVersion WHERE Title='einstein_platform' AND IsLatest = true];
System.debug('##cv'+cv);
String keyContents = cv.VersionData.toString();
keyContents = keyContents.replace('-----BEGIN RSA PRIVATE KEY-----', '');
keyContents = keyContents.replace('-----END RSA PRIVATE KEY-----', '');
keyContents = keyContents.replace('\n','');

JWT jwt = new JWT('RS256');
jwt.pkcs8 = keyContents;
jwt.iss = 'developer.force.com';
jwt.sub = 'abc@efg.com';
jwt.aud = 'https://api.einstein.ai/v2/oauth2/token';
jwt.exp = '3600';
String access_token = JWTBearerFlow.getAccessToken('https://api.einstein.ai/v2/oauth2/token', jwt);
System.debug('##access_token'+access_token);

 

public class JWT {
    
    public String alg {get;set;}
    public String iss {get;set;}
    public String sub {get;set;}
    public String aud {get;set;}
    public String exp {get;set;}
    public String iat {get;set;}
    public Map<String,String> claims {get;set;}
    public Integer validFor {get;set;}
    public String cert {get;set;}
    public String pkcs8 {get;set;}
    public String privateKey {get;set;}
    
    
    public static final String HS256 = 'HS256';
    public static final String RS256 = 'RS256';
    public static final String NONE = 'none';

    
    public JWT(String alg) {
        this.alg = alg;
        this.validFor = 300;
    }
    
    
    public String issue() {
    
        String jwt = '';
    
        JSONGenerator header = JSON.createGenerator(false);
        header.writeStartObject();
        header.writeStringField('alg', this.alg);
        header.writeEndObject();
        String encodedHeader = base64URLencode(Blob.valueOf(header.getAsString()));
            
        JSONGenerator body = JSON.createGenerator(false);
        body.writeStartObject();
        body.writeStringField('iss', this.iss);
        body.writeStringField('sub', this.sub);
        body.writeStringField('aud', this.aud);
        Long rightNow = (dateTime.now().getTime()/1000)+1;
        body.writeNumberField('iat', rightNow);
        body.writeNumberField('exp', (rightNow + validFor));
        if (claims != null) {
            for (String claim : claims.keySet()) {
                body.writeStringField(claim, claims.get(claim));
            }
        }
        body.writeEndObject();
        
        jwt = encodedHeader + '.' + base64URLencode(Blob.valueOf(body.getAsString()));
        
        if ( this.alg == HS256 ) {
            Blob key = EncodingUtil.base64Decode(privateKey);
            Blob signature = Crypto.generateMac('hmacSHA256',Blob.valueof(jwt),key);
            jwt += '.' + base64URLencode(signature);  
        } else if ( this.alg == RS256 ) {
            Blob signature = null;
            
            if (cert != null ) {
                signature = Crypto.signWithCertificate('rsa-sha256', Blob.valueOf(jwt), cert);
            } else {
                Blob privateKey = EncodingUtil.base64Decode(pkcs8);
                signature = Crypto.sign('rsa-sha256', Blob.valueOf(jwt), privateKey);
            }
            jwt += '.' + base64URLencode(signature);  
        } else if ( this.alg == NONE ) {
            jwt += '.';
        }
        
        return jwt;
    
    }
    

    public String base64URLencode(Blob input){ 
        String output = encodingUtil.base64Encode(input);
        output = output.replace('+', '-');
        output = output.replace('/', '_');
        while ( output.endsWith('=')){
            output = output.subString(0,output.length()-1);
        }
        return output;
    }
    

}

 

public class JWTBearerFlow {

    public static String getAccessToken(String tokenEndpoint, JWT jwt) {
    
        String access_token = null;
        try{
            String body = 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=' + jwt.issue();
            HttpRequest req = new HttpRequest();                            
            req.setMethod('POST');
            req.setEndpoint(tokenEndpoint);
            req.setHeader('Content-type', 'application/x-www-form-urlencoded');
            req.setBody(body);
            Http http = new Http();               
            HTTPResponse res = http.send(req);
            System.debug('###res:'+res);
            System.debug('###getStatusCode:'+res.getStatusCode());
            if ( res.getStatusCode() == 200 ) {
                System.JSONParser parser = System.JSON.createParser(res.getBody());
                while (parser.nextToken() != null) {
                    if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && (parser.getText() == 'access_token')) {
                        parser.nextToken();
                        access_token = parser.getText();
                        break;
                    }
                }
            }
        }catch(Exception ex){
            System.debug('## Exception '+ ex.getMessage() + ex.getStackTraceString());
        }
        
        return access_token;
        
    }

}

I am getting response with status as Forbidden and error code 403. So please help me through this.