When attempting to start certain missions, if you do not have a suitable weapon already equipped, you will see a prompt explaining what is recommended, or sometimes required to start a mission. If you then confirm the prompt, you will be taken to the Weapon screen and then presented options depending on what is recommended/required.
If you need to equip a certain weapon that you already own, just simply tap the “Equip” button when you are brought to the Weapon screen and then tap “Start”.
If you need to use a weapon that you do not own yet, you will need to purchase it via one of the payment methods shown on the right of the screen before equipping it.
If you need to upgrade weapon parts, tapping the "Start" button will bring up a notice indicating the part/s that need upgrading. If you already own a weapon that meets the recommended/required parts, you will be presented the option to equip it with a blue button in the bottom-right corner. If you don't own a weapon that meets the recommended/required parts, the blue button will take you to the weapon part that needs upgrading. Once again, you will be shown a payment method to the right of the screen.
If a mission requires you to equip/upgrade a weapon to start a mission (e.g. Assault Rifle/Shotgun for Contract missions), you will not be able to start the mission until you have the correct weapon equipped.
If a mission simply recommends a weapon to be equipped/upgraded, you may choose to proceed without doing this. However, doing so will make the mission more difficult, and continuing to ignore the recommendations may lead to certain missions becoming near impossible to complete. It is advised to meet the recommendations when possible for a smoother playing experience.