When we use MUST for an obligation, we cannot follow it with ‘can’ we use ‘be able to’ instead.

“When you climb a mountain you must be able to walk a long way”.

→ To use the pool at the hotel, you must be able to swim.

→ Applicants for this job must be able to speak English.

→ Our new system must be able to process all the orders we get.

→ The new secretary must be able to organise the office well.