Combining ±% with ±dB in measurement uncertainty
Never mind that one scale is logarithmic and the other is linear. Both error specs indicate ratios. I'll assume you're measuring power in some form.

Then an error of +/- 0.5 dB indicates the true value is between 0.89x and 1.12x what you measured (with whatever certainty is assumed by your definition of uncertainty).

And an error of +/- 5% indicates the value is between 0.95x and 1.05x what you measured.

If both uncertainties apply to the same measurement, you can multiply these ratios together and find the overall range is 0.846x to 1.178x the measurement, or that the overall error is +17.8% / -15.4% or +0.71 / -0.62 dB.

If you're measuring amplitudes rather than powers, of course you'll need to convert between dB and ratios differently.