Renegotiate your role, pay and especially your title if they want you to take charge. Be vocal to them about your skills, what you can, cannot do and what you feel comfortable with. Express you are keen to learn through proper training programs/mentoring and keen to take on more responsibilities. You're showing you're willing to support them, so they should support you. They should be able to identify if you're fit for the role, they're managers so it's their job.

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Try not to make their problems your own. You're fulfilling your job role and that's what you're contracted to do. Don't feel pressured to take on responsibilities outside of your contract, all you can do is show enthusiasm and a willingness to improve on yourself.