Code

const msg = await message.reply({content: 'Pulsa el botón para empezar a rellenar el formulario.', components: [btnRow]});
const collector = msg.createMessageComponentCollector();
collector.on("collect", async (i) => {
    showModal(modal, {client: client, interaction: i});
    msg.delete();     // I've tried removing this
    message.delete(); // and this line.
});

client.on('modalSubmit', async (modal) => {
    if (modal.customId === 'deberes') {
        const dia = modal.getTextInputValue('dia');
        const accion = modal.getTextInputValue('accion');
        const cosas = modal.getTextInputValue('cosas');

        modal.reply(`${dia} ... ${accion} ... ${cosas}`);
    }
});