Do you need a Radial J48 to record guitar into a Scarlett Solo?
If the guitar is on your desk, no: the Solo's instrument input already does the impedance conversion the DI exists for. And an active DI hands the preamp a signal 6 dB quieter than the cable did.
Short answer: for a guitar three metres from the desk, no. The Solo's instrument input is the DI box, built in.
A magnetic pickup is a high-impedance source. Feed it into a low-impedance input and it loses treble and level, which is what a DI box exists to prevent: it converts high impedance to low and unbalanced to balanced. The Scarlett Solo's second input does the first half of that job when INST mode is engaged — the input becomes high-impedance and the pickup is happy.
The numbers make the rest of the case. Straight into the instrument input, a guitar asks the preamp for roughly 20 dB. Through the J48 the requirement becomes about 26 dB, because this database lists the J48 at −6 dB of level. The Solo 4th Gen has 57 dB, so either way you are nowhere near a limit — but you have spent 239 dollars to make the preamp work harder.
The thing the word "active" hides: active means powered, not louder. The J48 draws +48 V phantom to run its input stage and still hands the preamp a quieter signal than the bare cable did. What the phantom buys is headroom and input impedance high enough not to load a hot active bass — not gain.
Where a J48 genuinely earns its price: a long run. High-impedance unbalanced cable loses treble with length because the cable's capacitance loads the pickup, and past roughly six metres it is audible. A DI at the instrument turns the rest of the journey into a balanced low-impedance line that can run thirty metres without loss, and its ground lift solves stage hum. On a desk, none of that applies.